Adding multi switch support to the Cisco Nexus plugin

This commit adds intelligent multiple nexus physical switch support for the Cisco plugin.
The plugin also has been modified to scan for the host when an instance is created and
selectively trunk VLAN's for the port for that host only. It also deletes VLANs from nexus
switches when no longer required.

Implements: blueprint cisco-plugin-enhancements

Change-Id: I6275eb1815310d0d5a8123ca2edbc0a0937718e9