Add release branching to workflows

[Mierdin]

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.