BGP: Fix MD5 authentication for unnumbered neighbors

BGP: Fix MD5 authentication for unnumbered neighbors

Ticket: CM-6369
Reviewed By: CCR-3318
Testing Done: Manual testing of various password scenarios.

This is a port of patch bgpd-unnumbered-nbr-fix-password.patch from
2.5-br.

In the case of BGP unnumbered, the peer IP address is derived and not
explicitly configured. If there is a password configured for the peer,
it can be set on the listen socket only after the IP address has been
derived and needs to be cleared when the IP address goes away.