[bgpcfgd]: Clarify error messages on reset Loopback0 ip address

[pavel-shirshov]

- Why I did it
To clarify error messages in case the ip address for Loopback is already set. It doesn't make sense to call correct ip address as ambiguous in this case

- How I did it
I split the check of constraints into two if clauses. One to check if the ip is correct or not. The second one to check if the ip address was already registered or not.

- How to verify it

admin@str-s6100-acs-1:~$ redis-cli 
127.0.0.1:6379> select 4
OK
127.0.0.1:6379[4]> keys LOOP*
1) "LOOPBACK_INTERFACE|Loopback0|10.1.0.32/32"
2) "LOOPBACK_INTERFACE|Loopback0"
3) "LOOPBACK_INTERFACE|Loopback0|FC00:1::32/128"
127.0.0.1:6379[4]> hset "LOOPBACK_INTERFACE|Loopback0|10.1.0.32/32" NULL NULL
(integer) 0
127.0.0.1:6379[4]> hset "LOOPBACK_INTERFACE|Loopback0|FC00:1::32/128" NULL NULL
(integer) 0

Jul 30 00:23:02.840009 str-s6100-acs-1 WARNING bgp#bgpcfgd: Update command is not supported for set src templates. current ip='10.1.0.32'. new ip='10.1.0.32'

Jul 30 00:23:28.543761 str-s6100-acs-1 WARNING bgp#bgpcfgd: Update command is not supported for set src templates. current ip='FC00:1::32'. new ip='FC00:1::32'

- Which release branch to backport (provide reason below if seleted)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

retest vs please

[pavel-shirshov]

retest vs please