feat(restapi): add ability to use any arbitrary postgres DB

[arschles]

not just RDS

Fixes #141
Closes #158

[krancour]

@arschles what database does this use out-of-the-box? i.e. controller, by default, uses the on-cluster postgres in deis-database. What does this use? I'm asking because I think we should be conscious of making off-cluster db configuration as easy for this as it is for the controller. (i.e. easily done via generate_params.toml).

[arschles]

@krancour this uses RDS out of the box (as of a9085d8), and in that case requires only the WORKFLOW_MANAGER_API_RDS_REGION env var. It gets the rest of the DB connection information from the AWS API.

Also, sidenote - this component has no helm chart - everything is deployed & configured via deis itself.

[arschles]

Also note that in the in-cluster case, this patch will make wfm-api require a secret, so we'll have to add a helm chart or similar

[arschles]

Removing from the 2.2 milestone, as it was never a necessary deliverable for that milestone

[mboersma]

I'm closing this PR since it's a year old 🎂, but @arschles please re-open if the code here is still relevant.