Improve registry.sh script UX

[miguelvr]

With the recent GCP merge the make image-<type>-<provider> commands require that all provider specific flags are set in dev/config/env.sh. They can currently be set as empty, but this seems unnecessary.

This PR improves the UX in 2 ways:

• If building for a specific provider, only that provider's env variables are required (the others do not need to be set)
• Add validation to check if provider specific env variables are set

checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)
• update examples
• update docs and add any new files to summary.md (view in gitbook after merging)
• cherry-pick into release branches if applicable
• alert the dev team if the dev environment changed

[RobertLucian]

LGTM with the exception of 2 nits.

[miguelvr]

@RobertLucian addressed your comments on my last commit