[rackspace|compute] updated mocking framework to support any flavor or image

I updated MockData to create additional images/flavors if they are unknown. Hopefully, this should allow developers to utilize mocks without having to change their code.

This should address issue https://github.com/fog/fog/issues/1573. 

Users wanting should use contain Fog::Rackspace::MockData::NOT_FOUND_ID for situations where it is desirable not to find an image or flavor.