Fix the format problem of LOG in failover_host
Fix the format problem of LOG in failover_host

In the failover_host function in manager.py. it misses
the 's' after %(backend_id), so if the driver throws exception,
it will throw another one too.

Change-Id: Ie676f3e0fb76a85c752d0e8dbe74bd9b214e4d0d
Closes-Bug: 1559399

NetApp/cinder,openstack/cinder,mahak/cinder,ge0rgi/cinder,j-griffith/cinder,Datera/cinder,Nexenta/cinder,mahak/cinder,cloudbase/cinder,eharney/cinder,cloudbase/cinder,bswartz/cinder,NetApp/cinder,phenoxim/cinder,bswartz/cinder,phenoxim/cinder,j-griffith/cinder,Datera/cinder,openstack/cinder,Hybrid-Cloud/cinder,Hybrid-Cloud/cinder,Nexenta/cinder,eharney/cinder