VMware: Fix ensure_vlan_bridge to work properly with existing DVS

With Nova Flat Network, if the DVS exists, we should not try to
worry about virtual switches or port groups. We should just return
the network_ref

ensure_vlan_bridge was also missing the return of network_ref at
tne end of the method, added that as well since that return value
is needed/used in virt/vmwareapi/vmops.py#L191

Fixes LP# 1194018

Change-Id: I78c71e4c15f1aa87378ae2eb0448c047c6a2a666
Merge "VMware: Fix ensure_vlan_bridge to work properly with existing DVS"