Switch failover-host from rpc call to cast

Switch failover-host from rpc call to cast

There is some concern that with large numbers of volumes it will be
difficult for drivers to failover the host before the rpc timeout hits.

To avoid asking admins to bump the timeout just for these cases we can
switch it to do a non-blocking cast instead of call. The difference now
being that the active_backend_id is not returned from the API call to
failover-host. An admin will have to look at the service-list output
to see when it has changed states from ‘failing-over’ and then check
what its active_backend_id is at that time.

Change-Id: I69b4908fe783cf785d3e1612422fca15fea01c6f
Closes-Bug: #1555342