bgpd: server socket is created for all enabled VRF

bgpd: server socket is created for all enabled VRF

Upon creation of BGP instances, server socket may or may not be created.
In the case of VRF instances, if the VRF backend relies on NETNS, then
a new server socket will be created for each BGP VRF instance. If the
VRF backend relies on VRF LITE, then only one server socket will be
enough. Moreover, At startup, with BGP VRF configuration, a server
socket may not be created if VRF is not the default one or VRF is not
recognized yet.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>