Complete the helm mixin schema

[carolynvs]

• [x] Compliant with new plans for how to embed the entire schema doc into the porter mega manifest.
  • Use an id
  • references use the full path rooted at the id
  • pass in the wrapping action, not just the step (so that we aren't forcing mixins to maintain separate json schema documents and then merge them for the schema command).
• [x] Validates the input yaml against the json schema during helm run.
• [x] Write a test that validates the mixin schema against a yaml file.

Incidentally this PR also has the following changes:

• Switches from using packr2 build to go generate for consistency with the porter repo
• Deletes the generated *-packr.go files so we can stop dealing with them on merges/rebases.
• Moves the packr box to a field on the mixin, and ensures it is always initialized, for consistency with the porter repo.

Closes #21

[carolynvs]

Once this is reviewed and accepted, I'll write up a document on the docs site explaining how to write a schema document for a mixin using the explanatory comments in the review as a guide.