Add interface for FogProvider

[marcusn]

There should be an interface (FogProvider or simliar) that encapsulates all the case classes in the fog driver. Subclasses should be instantiated as a "provider" in the fog driver instance, where all provider specific logic is delegated. This should include the acquire ip stuff, which I believe is openstack specific?

This would make it much easier to add a new provider.

[jkeiser]

Agreed! FogDrivet.from URL could choose which class to instantiate based on the provider (like fog:AWS)