Epic: Support build-time credential injection

[vdice]

There are use cases where Porter authors would like a way to inject credentials at build-time (rather than the currently supported run-time injection). For instance, if a mixin supports extended configuration and this configuration may need credentials to run. (See getporter/helm2-mixin#61 for an example)

To enable this, we'd need the following features added:

• Porter build takes an optional credential set, porter build -c cred

• Credentials in a Porter manifest can be designated build-only, such that they are not included into the resulting bundle but are available to inject into build-time config. For example:

  credentials:
    - name: repo-username
      # credential is only available at buildtime
      buildtime: true 
      default: bobby-tables

• Build-only credentials in a Porter manifest can be interpolated (via {{ build.credentials.mybuildcred }}) in build-time config (for instance, mixin config). Note: here we use build.credentials vs bundle.credentials to distinguish build-only from bundle.

[carolynvs]

Closing and tracking under #1769