[SNMP:Contact] Fix for config replace failure with SNMP Contact issue

[Verma-Anukul]

==> Issue:
• Config replace/yang-validation fails for SNMP contact configuration.
• In SNMP contact CLI, email field is mandatory
Related to sonic-net/sonic-buildimage#12620

==> Root Cause:
• sonic-snmp.yang defines leaf Contact, but sonic-utilities (CLI backend) writes to ConfigDB with key {contact_name}. So YANG validation fails when the key is {contact_name} and not the string "Contact"
• CLI current implementation is expecting two mandatory arguments

==> Fix:
• sonic-utilities (CLI backend) code is updated to use Yang leaf name "Contact" string as key and user passed input as "value"
• CLI implementation (sonic-utilities) is changed to expect only a single string, where user can pass any string of choice (name, email, phone, address or combination of these). Accordingly, "show" CLIs will display only "contact" instead of "name" & "email"

• Migration script is updated to take care of config_db migration

Tests:

• Test CLIs manually
• Test config replace
• Test config migration

PRs :
sonic-buildimage - sonic-net/sonic-buildimage#19834
sonic-utilities - sonic-net/sonic-utilities#3476
SONiC (docs) - #1774

Signed-off-by: Anukul Verma anukverm@cisco.com