Problem: playbooks/archivematica doesn't explain how to deploy QA

As f80bf57aad47e166c28ad19a8a828b61b9547e82, if someone tries to deploy Archiveamtica using the deploy-pub/playbooks/archivematica it's unclear for a newcomer if stable or qa branches are being deployed. This just beat @mcantelon which is a sign that we're doing a poor job explaining it.

I suggest to have two environments:
- `deploy-pub/playbooks/archivematica`
- `deploy-pub/playbooks/archivematica-qa`

Or we could do a better job in the README file explaning how to download the right branch of the AM role (e.g. qa/1.7.x branch targets AM 1.7 and so on) and the right vars file.

Read deploy-pub/playbooks/archivematica and the README it references, then inspect how the ArchiveMatic role branch and vars file are selected. Decide whether separate QA playbooks or clearer README guidance best fits the repository; done means a newcomer can distinguish stable from QA deployment and select the matching branch and vars.

Written by the indexing model from the issue text.