*: add a vrf update hook to be informed of the vrf name

*: add a vrf update hook to be informed of the vrf name

The Vrf aliases can be known with a specific hook. That hook will then,
from zebra propagate the information to the relevant zapi clients.
The registration hook function is the same for all daemons.

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