First pass at the emr module. I want to start reviewing this and at the same time, work with the LS team to
test this in a real scenario.

Known missing:
- No terminate in place
- Replace subnet ID with VPC ID and randomly choose a subnet
- No tags for the cluster.
Fixing connection issue
From review: set visibility to True by default, and require a jobflow role.
From review comments: using positional and **kwargs to streamline and simplify argument handling.
Merge pull request #2 from dataxu/dev-sarmit-emr-module

First pass at the emr module.