Remove the Cisco Nexus monolithic plugin support

The Cisco Nexus monolithic plugin does not work without the Open
vSwitch plugin. The Open vSwitch plugin is scheduled to be removed
as per #1323729. This patch removes the Nexus Hardware switch
related code from devstack. The N1KV virtual switch related code
will still remain in the tree as it doesn't depend on Open vSwitch
plugin.

Closes-Bug: #1350387
Change-Id: I82ebb09c64589fc9b7bb790982541bc87c66e6e3
Merge "Remove the Cisco Nexus monolithic plugin support"