Add interface to Firecloud

[jonn-smith]

@yfarjoun pointed out that a lot of the operations we do for cromshell have firecloud equivalents. Moreover, firecloud also has a nice swagger API.

I think we should add in the ability to talk to a firecloud server as well. The default would still be cromwell.

[lbergelson]

That sounds like a good idea. https://github.com/broadinstitute/fiss can probably help.