[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.
I'm a little suspicious about leaving out the `handle_*` functions in the contrail case. Also, the hook script below must be called in any case.

Also, to keep the contrail case more in line with the other two backends, it would be better to do any special casing in the `add_to_bridge` function.