ML2 Cisco Nexus MD - not overwriting existing config

The Cisco Nexus ML2 MD overwrites any existing switchport
VLAN config such as management VLANs that are preconfigured
in the compute node ToR interfaces.
This bug addresses that issue and ensures the config is not
wiped out.

Closes-Bug: 1330597

Change-Id: Ie2969e5750a179a4940d12db4c9ee6e1ec53b79b
Merge "ML2 Cisco Nexus MD - not overwriting existing config"