zebra: add hooks upon enabling / disabling a VRF

zebra: add hooks upon enabling / disabling a VRF

zebra_vrf_enable() is the callback for VRF_ENABLE_HOOK.
It presently needs do nothing.

zebra_vrf_disable() is the callback for VRF_DISABLE_HOOK.
It presently withdraws routes, shuts down interfaces, and
clears the router-id candidates in that VRF.

Signed-off-by: Feng Lu <lu.feng@6wind.com>
Reviewed-by: Alain Ritoux <alain.ritoux@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Vincent JARDIN <vincent.jardin@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>