bgpd: fix import vrf creates multiple bgp instances

bgpd: fix import vrf creates multiple bgp instances

The more the vrf green is referenced in the import bgp command, the more
there are instances created. The below configuration shows that the vrf
green is referenced twice, and two BGP instances of vrf green are
created.

The below configuration:
> router bgp 99
> [..]
>  import vrf green
> exit
> router bgp 99 vrf blue
> [..]
>  import vrf green
> exit
> router bgp 99 vrf green
> [..]
> exit
>
> r4# show bgp vrfs
> Type  Id     routerId          #PeersCfg  #PeersEstb  Name
>              L3-VNI            RouterMAC              Interface
> DFLT  0      10.0.3.4          0          0           default
>              0                 00:00:00:00:00:00      unknown
>  VRF  5      10.0.40.4         0          0           blue
>              0                 00:00:00:00:00:00      unknown
>  VRF  6      0.0.0.0           0          0           green
>              0                 00:00:00:00:00:00      unknown
>  VRF  6      10.0.94.4         0          0           green
>              0                 00:00:00:00:00:00      unknown

Fix this at import command, by looking at an already present bgp
instance.

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