Check that admin interface will not be assigned to vlan bridge in nsx mode

We have to be sure that admin interface will not be assigned to VLAN bridge in NSX mode, because we will lose connection to this node.

Without `br1` we lose only possibility to use VLAN's as a network backend, main transport protocol STT will work without any problems.
I'd add a comment here, specifying the reason of this check (basically what you explain in the pull request)