topotests: bgp_bmp, expose peer_distinguisher in loc-rib

topotests: bgp_bmp, expose peer_distinguisher in loc-rib

The BMP implementation currently only supports global and
loc-rib instance types. When loc-rib is selected, the
peer_distinguisher is set to the route distinguisher of
the L3VRF where the BGP instance is. This functionality has
not been tested until now, because the peer distinguisher
value had been explicitly omitted in the bmp messages.

Expose the peer distinguisher value in all BMP messages
received. This change requires to modify the expected output
for loc-rib when the BGP instance is in a L3VRF.

The handling of peer distinguisher value for RD instances
will follow in the next commits.

Link: https://www.rfc-editor.org/rfc/rfc7854.html#section-4.2
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>