Add GCE as a supported provider

[nchammas]

Flintrock was designed from the start to make it possible to support multiple cloud providers within one tool.

Google's Compute Engine (GCE) is most likely the next cloud provider that Flintrock will support.

It will take quite a bit of work to do this, and in the process Flintrock will be forced to refine its internals which will make it more maintainable in the long run. This is a good thing.

Work on this should probably start only after #6 and #8 have been resolved, so that there is a slightly better definition of the APIs and structure that the GCE provider can follow. #11 is another blocker for this issue.

[nchammas]

For those who may be watching this issue and waiting for GCE support, I recommend checking out @broxtronix's project, spark-gce.

[nchammas]

I'm going to close this for now since, realistically speaking, this is not going to happen without a motivated contributor 1) stepping up to add this and 2) committing to maintain it over the long run (which would entail becoming a Flintrock committer). That contributor is likely not going to be me. We can reopen this issue if such a contributor steps up.

I am still committed to maintaining an appropriate separation of concerns within Flintrock such that, if support for another provider like GCE is to be added, it can be done without too much surgery. Modules like core.py and services.py will continue to be completely provider-agnostic to make that possible.