feat(*): add Init logic to auto-init Tiller and/or sync client/server versions

[vdice]

• adds an Init function that will:
  • init Helm 2's server-side component (Tiller) if unresponsive
  • dynamically install a client version matching the Tiller version on the provided cluster, if not already in sync

If this approach is looking good, there are still a few potential TODOs:

TODO

• [x] add RBAC setup after init-ing Tiller?
• [x] refactor for unit testability?

Closes https://github.com/deislabs/porter-helm/issues/20

[carolynvs]

Oooh, doing the RBAC setup properly would be super useful, but would be perfect for a follow-up PR.