bgpd: Fix for CM-11908 BGP: 'Address-family encap' cli issues

bgpd: Fix for CM-11908 BGP: 'Address-family encap' cli issues

Made fix to handle the "Ambigious command" for address-family vpvn6 and vpnv6 unicast.
Rest of the bug analysis is below:
1. Issues with address-family encap/address-family encapv6/address-family vpnv6/address-family vpnv6 unicast need to be added to the ignore list in the test file tests.l3.quagga_cfg_cli_crawler_test. Sid to fix the "Ambiguous command" for vpnv6 as part of fix for this bug.
2. Neighbor <ipaddr/interface> disable-connected-check/ttl. Options should not be listed for interface. Anitha to file bugs for these. And also remove these commands from the test file tests.l3.quagga_cfg_cli_crawler_test.

Ticket:  CM-11908
Reviewed By: CCR-4999
Testing Done: Manual, Ran the tests.l3.quagga_cfg_cli_crawler_test

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