Fix mgmt-framework rest server startup script

[sachinholla]

Why I did it

Mgmt-framework REST startup script is broken after PR #4937. This PR misspells the table name as "RESET_SERVER"; hence the user defined settings from REST_SERVER table are never loaded. REST server always comes up with default settings.
Also the startup script rest-server.sh does not use the jq -r commands correctly. With -r option, it prints a null value when the field requested does not exists. Startup script would have assigned a "null" value for all non-configured parameters and would have failed (if it had spelt the table name correctly in mgmt_vars.j2).

How I did it

Fixed mgmt_vars.j2 to load correct db entry and rest-server.sh to handle optional fields correctly.

Also introduced a new jinja filter "json" in sonic-cfggen, which formats the value using json.dumps() function. The mgmt_vars.j2 template uses this filter to transform db values into a syntactically correct json.

How to verify it

Verify REST server startup and a GET operation with following configurations:

• No REST_SERVER and x509 table entries
• With client_auth config in REST_SERVER entry
• With log_level config in REST_SERVER entry
• With certificate configs in REST_SERVER entry
• WIth all configs in REST_SERVER entry
• With certificate configs in x509 entry
• With client_auth config in REST_SERVER entry and certificate configs in x509 entry

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog