Prevent configuring IP interface on a port which is a member of VLAN …

…(#1374)

**- What I did**
Fixed bug sonic-net/sonic-buildimage#6426
Added a validation in config/main.py to prevent configuring IP interface on a port which is a member of VLAN

**- How I did it**
Change config/main.py

**- How to verify it**
Add interface as member in vlan
Try to configure IP address on same interface

**- Previous command output (if the output of a command-line utility has changed)**
Success in the above scenario. 

**- New command output (if the output of a command-line utility has changed)**
prevent configuration:
"Interface Ethernet0 is a member of vlan
Aborting!"

config/main.py

@@ -2645,6 +2645,13 @@ def add(ctx, interface_name, ip_addr, gw):
         if interface_name is None:
             ctx.fail("'interface_name' is None!")
 
+    # Add a validation to check this interface is not a member in vlan before 
+    # changing it to a router port 
+    vlan_member_table = config_db.get_table('VLAN_MEMBER')
+    if (interface_is_in_vlan(vlan_member_table, interface_name)):
+            click.echo("Interface {} is a member of vlan\nAborting!".format(interface_name))
+            return
+
     try:
         net = ipaddress.ip_network(ip_addr, strict=False)
         if '/' not in ip_addr: