Draft: allow VXLAN connections without an explicit remote VTEP

EVPN/VXLAN dynamically learns remote VTEPs using BGP; see [https://vincent.bernat.ch/en/blog/2017-vxlan-bgp-evpn#vtep-setup] for a recipe.

This patch allows the remote property not to be set explicitly in NetworkManager, to allow the EVPN use case.