Replication disabled reason for replication enabled backends.

During initialize of replication backend which consists of primary site and secondary site because of some reason any of the site may go down and in cinder service replication_status is set to disabled.

So to give information to user why replication is disabled, service should be updated with disabled_reason.

This implementation involves:
1. Introducing the new field in service to store the disabled reason information.
2. Code changes in "cinder/volume/manager.py" file.