Disable nova-compute on libvirt connectivity exceptions

The service will be disabled and re-enabled according to the success
or failure of establishing a connection to the libvirt service.

update_available_resources, periodic task, will probe the connection
even if the service is disabled.

This is in order to avoid migrations or new instances to be scheduled
on a disconnected from libvirt compute node.

Closes-Bug: #1240670
Change-Id: Ib8d67838ceb73c5b1cdc9498c17b335e9e5bb6f3
Merge "Disable nova-compute on libvirt connectivity exceptions"