engine: Fixes rollback handling in ChangeVDSClusterCommand

Fixes rollback handling in ChangeVDSClusterCommand, so when an execption
is occured during command, there will not be created two rows for the
host in Hosts list

Change-Id: I1866901d73b1a3270d51b7b261ce5acf0ea67ce9
Bug-Url: https://bugzilla.redhat.com/984927
Signed-off-by: Martin Perina <mperina@redhat.com>