[master]fix #4716 show ipv6 interfaces neighbor_ip is N/A issue

[tim-rj]

• Why I did it

  When exec 'show ipv6 interfaces', the neighbor_ip will always be 'N/A' which will confuse the consumers. After checking we found that this is a show issue. As we know, there will be one local linked IPV6 address for every valid interface. In show/main.py, the interface's ipv6 neighbor ip will always be written by this local linked IPV6's neighbor_ip address which is 'N/A'. And i think this part code of 'show ipv6 interface' was copied from 'show ip interface' before, right? Yea, some change is necessary!

• What I did

  This is bug fixing. Using a array to store neighbor info in case that the old one be written by the newer one.

• How I did it

  Code change.

• How to verify it
  Upgrate config_db.config attached in show ipv6 interfaces command result doesn't conclude neighbor ipv6 address sonic-buildimage#4716 and show ipv6 interfaces

• Previous command output (if the output of a command-line utility has changed)

root@switch2:/usr/lib/python2.7/dist-packages/show# show ipv6 interfaces
Interface     Master        IPv6 address/mask                          Admin/Oper    BGP Neighbor    Neighbor IP
------------  ------------  -----------------------------------------  ------------  --------------  -------------
Bridge                      fe80::fce5:f4ff:fed5:b262%Bridge/64        up/up         N/A             N/A
Ethernet0     PortChannel1  fe80::5a69:6cff:fefb:2230%Ethernet0/64     up/up         N/A             N/A
Ethernet52                  2001:50:5:1::5/64                          up/up         N/A             N/A
                            2001:50:5:1::4/64
                            2001:50:5:1::3/64
                            2001:50:5:1::2/64
                            fe80::5a69:6cff:fefb:2230%Ethernet52/64
Loopback0                   6::6/128                                   up/up         N/A             N/A
                            fe80::44cb:61ff:fe6b:4d41%Loopback0/64
PortChannel1  Bridge        fe80::5a69:6cff:fefb:2230%PortChannel1/64  up/up         N/A             N/A
Vlan3                       150::1/64                                  up/up         N/A             N/A
                            fe80::5a69:6cff:fefb:2230%Vlan3/64
eth0                        fe80::5a69:6cff:fefb:2230%eth0/64          up/up         N/A             N/A
lo                          ::1/128                                    up/up         N/A             N/A
root@switch2:/usr/lib/python2.7/dist-packages/show#

• New command output (if the output of a command-line utility has changed)

root@switch2:/home/admin# show ipv6 interfaces
Interface     Master        IPv6 address/mask                          Admin/Oper    BGP Neighbor    Neighbor IP
------------  ------------  -----------------------------------------  ------------  --------------  --------------
Bridge                      fe80::fce5:f4ff:fed5:b262%Bridge/64        up/up         N/A             N/A
Ethernet0     PortChannel1  fe80::5a69:6cff:fefb:2230%Ethernet0/64     up/up         N/A             N/A
Ethernet52                  2001:50:5:1::5/64                          up/up         LCv6            2001:50:5:1::9
                            2001:50:5:1::4/64                          up/up         LCv6            2001:50:5:1::8
                            2001:50:5:1::3/64                          up/up         LCv6            2001:50:5:1::7
                            2001:50:5:1::2/64                          up/up         LCv6            2001:50:5:1::1
                            fe80::5a69:6cff:fefb:2230%Ethernet52/64    up/up         N/A             N/A
Loopback0                   6::6/128                                   up/up         N/A             N/A
                            fe80::44cb:61ff:fe6b:4d41%Loopback0/64     up/up         N/A             N/A
PortChannel1  Bridge        fe80::5a69:6cff:fefb:2230%PortChannel1/64  up/up         N/A             N/A
Vlan3                       150::1/64                                  up/up         N/A             N/A
                            fe80::5a69:6cff:fefb:2230%Vlan3/64         up/up         N/A             N/A
eth0                        fe80::5a69:6cff:fefb:2230%eth0/64          up/up         N/A             N/A
lo                          ::1/128                                    up/up         N/A             N/A

[tim-rj]

@prsunny as mentioned in #938, this issue exists in master branch also, I create pull request for master branch here. Please check check_log below:
check_log.txt

[prsunny]

@tim-rj, can you provide the previous show output and the new output as required by the description template? Also please use the code format for properly aligning the output

[tim-rj]

@prsunny I have updated 'Previous command output' and 'New command output ' above, please help check again. (If want to check the neighbor info correct or not , please check file->
check_log.txt
)

[prsunny]

Isn't it better to have the mapping based on the ipaddr (dictionary) instead of an implicit index? Just a thought!

[tim-rj]

Please help arrange merging, thanks!