dummy: T2241: fix falling out of bridge when changing settings
dummy: T2241: fix falling out of bridge when changing settings

Previously, set_vrf was always called, which uses the same master and nomaster
commands as bridge, so it removed the interface from the bridge.
- add checks to make VRF and bridge membership mutually exclusive

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x