Get the OSA sha referenced by an RPCO Repo.

def get_osa_commit(repo, ref, rpc_product=None):
    """Get the OSA sha referenced by an RPCO Repo."""
    osa_differ.checkout(repo, ref)

    functions_path = os.path.join(repo.working_tree_dir,
                                  'scripts/functions.sh')
    release_path = os.path.join(repo.working_tree_dir,
                                'playbooks/vars/rpc-release.yml')

    if os.path.exists(release_path):
        with open(release_path) as f:
            rpc_release_data = yaml.safe_load(f.read())

        rpc_product_releases = rpc_release_data['rpc_product_releases']
        release_data = rpc_product_releases[rpc_product]

        return release_data['osa_release']

    elif repo.submodules['openstack-ansible']:
        return repo.submodules['openstack-ansible'].hexsha

    elif os.path.exists(functions_path):
        # This branch doesn't use a submodule for OSA
        # Pull the SHA out of functions.sh
        quoted_re = re.compile('OSA_RELEASE:-?"?([^"}]+)["}]')
        with open(functions_path, "r") as funcs:
            for line in funcs.readlines():
                match = quoted_re.search(line)
                if match:
                    return match.groups()[0]
            else:
                raise SHANotFound(
                    ("Cannot find OSA SHA in submodule or "
                     "script: {}".format(functions_path)))

    else:
        raise SHANotFound('No OSA SHA was able to be derived.')