BFV - no image validation, integration tests

Broke this off from https://github.com/rackerlabs/otter/pull/711 because it was starting to get to be a large PR.

Can't write tests for not providing the image ID param at all, because that would probably require a refactoring on the part of the cloudcafe CreateServer request object.

TODO:
- [x] system test that just depends on mimic right now
What happens on specification of block_device_mapping and a non-empty image_id?