Rackspace Cloud Databases disk usage plugin

Plugin to monitor Rackspace Cloud Databases disk usage.

```bash
$ python clouddatabase_size.py 1f375748-3abb-4c77-849a-2611a185818f ORD
status ok usage within threshold
metric database_GB_container_size float 5
metric database_GB_used float 0.244094848633
metric percentage_used float 0.0488189697266
```