What is the 'root' location for playbooks?
What about using roles inside playbooks? Will e.g. requirements.yml be supported (as in ansible-galaxy install -r requirements.yml)

Good points! Thanks, @lzachar. I would suggest to keep the ansible playbook root consisent with the metadata tree root. That would be, for the ci.fmf case in dist git, root of the dist git repository. For example:

prepare:
    how: ansible
    playbooks:
        - tests/common.yml
        - tests/rhel7.yml

Regarding extra roles: I think we could provide an easy way to specify which roles should be installed from the ansible galaxy before playbooks are executed:

prepare:
    how: ansible
    roles:
        - nginxinc.nginx
    playbooks:
        - web.yml

Would something like this work for you?

Regarding extra roles: I think we could provide an easy way to specify which roles should be installed from the ansible galaxy before playbooks are executed:
prepare:
how: ansible
roles:
- nginxinc.nginx

Disclaimer - I'm not power user of roles - but you can specify SCM in https://docs.ansible.com/ansible/latest/reference_appendices/galaxy.html#installing-multiple-roles-from-a-file
Which looks like very handy feature.
How about keeping same functionality? In simplest case we could dump them into requirements.yml and let ansible-galaxy to take care of it.

I'd say we start with the simple use case only or support both role names and requirement files under a single roles attribute.

+1 for roles containing (possible) mixture of role names, git URLs or files with requirements.

Thanks for the feedback. Updated.

I like it like this for the start, so +1 from me