Config interface Loopback0 ip add/del is not working (#443)

* Incremental addition/deletion of address on Loopback0 interface are not working.

Fix:
  - Check if add/remove are called on Loopback0 interface and pass them to config DB.

Author: kirankella

config/main.py

@@ -952,6 +952,9 @@ def add(ctx, interface_name, ip_addr):
         config_db.set_entry("PORTCHANNEL_INTERFACE", (interface_name, ip_addr), {"NULL": "NULL"})
     elif interface_name.startswith("Vlan"):
         config_db.set_entry("VLAN_INTERFACE", (interface_name, ip_addr), {"NULL": "NULL"})
+    elif interface_name.startswith("Loopback"):
+        config_db.set_entry("LOOPBACK_INTERFACE", (interface_name, ip_addr), {"NULL": "NULL"})
+
 
 #
 # 'del' subcommand
@@ -975,6 +978,8 @@ def remove(ctx, interface_name, ip_addr):
         config_db.set_entry("PORTCHANNEL_INTERFACE", (interface_name, ip_addr), None)
     elif interface_name.startswith("Vlan"):
         config_db.set_entry("VLAN_INTERFACE", (interface_name, ip_addr), None)
+    elif interface_name.startswith("Loopback"):
+        config_db.set_entry("LOOPBACK_INTERFACE", (interface_name, ip_addr), None)
 
 #
 # 'acl' group ('config acl ...')