bgpd: implement bgpPeerTable accross VRFs

bgpd: implement bgpPeerTable accross VRFs

Currently, bgpPeerTable only looks the default BGP instance. Most
vendors return all the available peers in this table. This commit
exposes all BGP instances.

The other tables are unchanged as it doesn't make sense to expose
routes from random VRFs into a single table. Vendors are using SNMP
contexts for that but we don't have support for it. Therefore, do
nothing.

Fix #6077

Signed-off-by: Vincent Bernat <vincent@bernat.ch>