Enable security group extension in PLUMgrid plugin

Implements: blueprint plumgrid-neutron-security-groups
Change-Id: I30392adff5e3250a1c4f9f1f04fc7e0587007226
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
Update Horizon Neutron-related settings

- Horizon no longer has "enable_security_group" setting
  so we need to remove it.
- There is no need to set enable_lb/firewall/vpn to True
  when q-lbaas/q-fwaas/q-vpn is enabled because Horizon now checks if
  Neutron ext-list and enables corresponding dashboards accordingly.

Change-Id: I37073d73e4cba0103ab1a3d935302f1cd0ef73c5
Merge "Update Horizon Neutron-related settings"
Merge "Remove the Cisco Nexus monolithic plugin support"
Merge "Enable security group extension in PLUMgrid plugin"