[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.
Also VLANs are not handled here; are they not supported by the Contrail VRouter? Or is the intention to add this later? Again, it would be best to raise `Not_implemented` (if `vlan <> None`).