vrf: return vrf implementation for default vrf

vrf: return vrf implementation for default vrf

To correct potential crash with netns implementation of vrf (see next
commit) it is necessary to allow any daemons to know the vrf
implementation whatever the vrf.
With current implementation the daemons do not know the vrf
implementation for the default vrf. For this vrf the returned vrf
implementation is always vrf-lite.
To solve this issue a netns name is set to the default vrf to just test
is presence to know the used implementation.

For zebra a netns name (if needed) is set in the vrf_init function just
before enabling the vrf. So this information is propagated to the other
daemons thanks the zapi message called when the vrf is enable at zebra
layer and override the default configuration (vrf-lite) of the daemon.

Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com>