vrf: T6189: render FRR L3VNI configuration when creating VRF instance

When adding and removing VRF instances on the fly it was noticed that the vni
statement under the VRF instance in FRR vanishes. This was caused by a race
condition which was previously designed to fix another bug.

The wierd design of a Python helper below the VRF tree to only generate the
VNI configuration nodes is now gone and all is rendered in the proper place.