SNMP input plugin: Fixed SNMPv3 encryption and authentication

[kauppine]

This PR fixes SNMP plugin's SNMPv3 authentication and encryption. Previously the parameters were set incorrectly, hence the requests were not using USM properly. Therefore, the requests were unencrypted and garbage values were observed as seen in #7746

This PR passes all the existing tests, however, in my opinion the SNMP-plugin would benefit from better unit tests.
I've used snmplabs' testing service: http://snmplabs.com/snmp-simulation-service.html#snmpv3-usm

Closes #7746
Using invalid auth or priv password yields an error:

[[inputs.snmp]]
agents = [ "demo.snmplabs.com:1161" ]
interval = "5s"
timeout = "5s"
retries = 3
version = 3
sec_name = "usr-md5-none"
auth_protocol = "MD5"
auth_password = "invalid"
sec_level = "authNoPriv"
 
  [[inputs.snmp.field]]
    name = "hostname"
    oid = ".1.3.6.1.2.1.1.5.0"
    is_tag = true
 
  [[inputs.snmp.field]]
    name = "uptime"
    oid = "1.3.6.1.2.1.1.3.0"
 
  [[inputs.snmp.field]]
    name = "cpmCPUTotal1min"
    oid = ".1.3.6.1.4.1.9.9.109.1.1.1.1.4.7"

./telegraf -test -config snmp.conf
2020-06-28T16:49:40Z I! Starting Telegraf
2020-06-28T16:49:42Z E! [inputs.snmp] Error in plugin: agent demo.snmplabs.com:1161: performing get on field hostname: Incoming packet is not authentic, discarding
2020-06-28T16:49:42Z E! [telegraf] Error running agent: input plugins recorded 1 errors

Closes #3788
Telegraf now sends messages encrypted if encryption is used:

[[inputs.snmp]]
agents = [ "demo.snmplabs.com:1161" ]
interval = "5s"
timeout = "5s"
retries = 3
version = 3
sec_name = "usr-md5-aes"
auth_protocol = "MD5"
auth_password = "authkey1"
priv_protocol = "AES"
priv_password = "privkey1"
sec_level = "authPriv"
 
  [[inputs.snmp.field]]
    name = "hostname"
    oid = ".1.3.6.1.2.1.1.5.0"
    is_tag = true
 
  [[inputs.snmp.field]]
    name = "uptime"
    oid = "1.3.6.1.2.1.1.3.0"
 
  [[inputs.snmp.field]]
    name = "cpmCPUTotal1min"
    oid = ".1.3.6.1.4.1.9.9.109.1.1.1.1.4.7"

./telegraf -test -config snmp.conf 
2020-06-28T17:08:08Z I! Starting Telegraf
snmp,agent_host=demo.snmplabs.com,hostname=458 uptime=1069527715i 1593364090000000000
> snmp,agent_host=demo.snmplabs.com,hostname=458 uptime=1069527715i 1593364090000000000

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

I must be missing something, I don't understand how these changes would have any effect as it seems like equivalent code. Can you explain how this works?