bgpd: Actually respect RFC 6286 for router_id

bgpd: Actually respect RFC 6286 for router_id

The RFC states:
      The BGP Identifier is a 4-octet, unsigned, non-zero integer that
      should be unique within an AS.  The value of the BGP Identifier
      for a BGP speaker is determined on startup and is the same for
      every local interface and every BGP peer.

We were going slightly beyond this and ensuring that the address
was a specific range of addresses which is no longer relevant.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>