Addl worker updates

[delgadom]

• [x] closes #3
• [x] tests added / passed
• [x] docs reflect changes
• [x] passes flake8 rhg_compute_tools tests docs
• [ ] entry in HISTORY.rst

This PR includes the following modifications:

• get_worker is renamed get_cluster
• Default parameters to get_cluster are pulled from worker_template.yml rather than overwritten. this is super important to allow for different cluster specifications on compute.rhg, compute-test.rhg, and compute.impactlab.
• Helper functions to create different cluster flavors are created (get_micro_cluster, get_standard_cluster, get_big_cluster, get_giant_cluster).

[delgadom]

Ok ready to request merging into #7