lib: Support auto completion of configured keychain.

lib: Support auto completion of configured keychain.

Problem Statement:
=================
When modules use keychain there is no option for auto completion
of configured keychains.

RCA:
====
Not implemented.

Fix:
====
Changes to support auto completion of configured keychain names.

Risk:
=====
Low risk

Tests Executed:
===============
Have tested auto completion of configured keychain names with newly
implemented auth CLI.

frr(config-if)# ipv6 ospf6 authentication keychain
  KEYCHAIN_NAME  Keychain name
     abcd pqr 12345

Signed-off-by: Abhinay Ramesh <rabhinay@vmware.com>