Feature request: Add support for database drop

[szwed]

Current implementation doesn't delete Atlas MongoDB databases, it deletes users only. For some use cases it may be a blocker.

It is a known issue as already mentioned in:
https://github.com/mickybart/python-atlasbroker/blob/master/atlasbroker/serviceinstance.py#L107

@mickybart we are considering adding the missing feature and pushing a PR, stay tuned..