Add new back-end parameter "always_failover"

Instances that have this parameter is set to True are never migrated, but
instead they can only fail over.  There are some cases where freezing the
kernel may cause problems, and hence this behavior is preferable.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>