Adds a test-master stage to the CI, which tests
bst-plugins-experimental against the latest commit to Buildstream
master. The intent is that we run this as a schedule daily(?) to check
for breakages to plugins from master. This job is allowed to fail.
Additionally adds test environments for this in tox, however running all of these resulted in the ugly changes to .gitlab-ci.yml. Of course, all but py37 are skipped in this docker image, but it might be nice to test at least py35 as well in CI.
Closes #10 (closed)