Daemon group based on restart test

base on https://github.com/autotest/autotest-docker/pull/255
How about squashing this into the following one to keep the number of conditionals down:

    if not self.config.get('subsubtests', ''):
        raise DockerTestNAError("Missing|empty 'subsubtests' in config.")
    self.subsubtest_names = config.get_as_list(self.config['subsubtests'])

Also, it's good to update things to use newer common functions as you come across them (no big deal though).