[CLEARWATER-SP1] Changes to enable contrailvrouter as a networking option in XenServer.

Changes to enable contrailvrouter as a networking option in XenServer.

* Changes to xcp-networkd and xapi daemons to accept contrailvrouter as a networking option in the
  /etc/xensource/network.conf file
* New module ContrailVR to handle commands that are received from XAPI daemon and pass it on to the
  corresponding Contrail Vrouter control module.
* Changes to script to change network backend.
* Changes in VIF script to handle contrailvrouter  to add, remove, online, offline VIFs
* contrail-vrouter-ctl which is the control component for the Contrail Vrouter is not part of this checkin and will be installed as part of the Contrail VRouter RPM.

Note: Please discard first two commits since it is a commit and a revert.
For performance reasons, this hotplug script should not contain any `xe` calls.