bgpd, zebra: Scope EVPN commands to EVPN VRF

bgpd, zebra: Scope EVPN commands to EVPN VRF

If the EVPN VRF is not the default one (i.e. with advertise-all-vni),
this allows showing its information with `show bgp l2evpn evpn ...`
commands. They do not require adding `vrf VRFNAME` since we only
support a single EVPN VRF. The same is true for zebra-specific commands
(e.g. `show evpn ...`).

Configuration commands are not restricted to the default VRF but to
the EVPN one, that is to the one bearing `advertise-all-vni`.

Signed-off-by: Tuetuopay <tuetuopay@me.com>
Sponsored-by: Scaleway