Missing action for old metrics cleanup

Ceilometer provides a script ceilometer-expirer that is responsible for cleaning the metrics older than the ttl specified in the ceilometer.conf file.
The charm currently doesn't support the ceilometer-expirer neither as an action of the charm nor as a cronjob to be executed daily.

Currently changing the retain period for metrics and events doesn't affect existing metrics.

An action to run ceilometer-expirer would be enough to be executed every time the retention period is updated through juju config.

This will solve the situation where a customer is running out of disk availability.