Allow specifying environment variables when generating a pod template spec

[whitlockjc]

Many applications use environment variables for runtime configuration and without the ability to specify environment variables when generating the PTS, generating a PTS is not very useful.

[noahdietz]

Environment variables will be specified by client at image build time and baked into the Dockerfile, pre-build, thus they are built into the image pulled by the PTS.

For documentation:

• these env variables should not contain sensitive information (i.e. secrets, tokens, etc) as they are insecure.
• PTS ("deployment-time") specified env variables will override duplicates specified in the Dockerfile ("build-time")

The other option was using the PTS URL as "storage" and encoding these env variables into the query string.