mongos-cfg-relation-broken hook does not exist in hooks.py

[Charles Clark]

When destroying a the mongos-cfg relationship, the charm fails. Even though there's an alias from hooks/mongos-cfg-relation-broken, there is no mongos-cfg-relation-broken in hooks.py. Here's the log file output:

unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 Traceback (most recent call last):
unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 File "/var/lib/juju/agents/unit-mongos-0/charm/hooks/mongos-cfg-relation-broken", line 1705, in <module>
unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 hooks.execute(sys.argv)
unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 File "/var/lib/juju/agents/unit-mongos-0/charm/hooks/charmhelpers/core/hookenv.py", line 550, in execute
unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 raise UnregisteredHookError(hook_name)
unit-mongos-0[903]: 2015-06-11 04:57:06 INFO unit.mongos/0.mongos-cfg-relation-broken logger.go:40 charmhelpers.core.hookenv.UnregisteredHookError: mongos-cfg-relation-broken
unit-mongos-0[903]: 2015-06-11 04:57:06 ERROR juju.worker.uniter.operation runhook.go:89 hook "mongos-cfg-relation-broken" failed: exit status 1