Add release branching to workflows

Right now, when workflows like `st2cd.st2_pkg_test_stable_u16` runs, it does so using production stable packages. However, the st2cd repo doesn't have release branches, so always the `master` branch is used there. So if someone is rolling out a new feature (i.e. Inquiries) and they write e2e tests for it, those tests will fail on prod because those packages don't have the feature.