Support to configure Cisco Nexus Plugin

Add support to configure a cisco nexus plugin as part of deploying Neutron via RDO/RHOS. This commit adds the following functionality:
* Expose necessary knobs to set up and configure the cisco plugin
  - Use either OVS (the default) or Cisco quantum plugin as Neutron core plugin
  - The nexus plugin also requires the OVS plugin to be set to 'vlan' mode
  - If cisco plugin, use OVS(the default) or n1k as virtualized l2
  - If using the Cisco plugin, Nexus hardware can be used for l2 (not configured by default)
  - If using the nexus sub plugin, specify the hardware layout
  - Set the nexus login credentials by creating a list of switch_ip/username/password strings
  - provider vlan auto create and removal options
* Logic to setup ovs/nexus settings and necessary plugins
* Logic to configure and deploy cisco_plugins.ini file for the plugin and switch configuration.