T6841: firewall: migrate existing VRF in zone based firewall

VRF support was introduced in VyOS 1.4.0. If a VRF is added as an interface in
the zone based firewall, it will be migrated to the new syntax.

OLD:

set firewall zone FOO interface RED
set firewall zone FOO interface eth0

NEW:

set firewall zone FOO member vrf RED
set firewall zone FOO member interface eth0