Provider Segment Support for Cisco Nexus Switches

This commit implements ML2 provider segment support
for the Cisco 3K, 5K and 7K Nexus switches.
Need to keep ToR provider segment information made by a network administrator
configured when the ML2 cisco_nexus mechanism driver is enabled.

Implements: Blueprint ml2-cisco-nexus-mechdriver-providernet
Change-Id: I43fa6fead8739c8099750f7ec0dacc9cf0a48f92
Merge "Provider Segment Support for Cisco Nexus Switches" into staging