tests/str: Rework invocation

tests/str: Rework invocation

Let's make our `run.sh` generically support any playbook. This is prep for
writing further tests in Ansible. Along with that, rework the Ansible so that
`tests.yml` is a playbook, and then the other bits are just task lists. It's
easier to read.

I also started to add a `use_git_build` variable with the idea that we'll be
able to run these same tests against an upstream image by setting that variable
off.

Closes: #1493
Approved by: jlebon