Integration with Ceilometer

It'll be great to have some ability to report/query metrics for Savanna.

There are two possible points of integration:

* report overall Savanna stats to Ceilometer (number of clusters/instances/volumes used overall/per-tenant/per-user/etc, size of clusters, used plugin/version, etc.)

* report metrics from each cluster (it could be implemented by adding new method to plugin that would be called to get some metrics, for example, number of mar/reduce tasks, health checks, hdfs free space, etc.)