Add role_names to valid keys entry for plays

Prevents a "not a legal parameter" error for play ds structures that
may be passed into the Play() constructor, which really is mainly a
concern during tests.

lib/ansible/playbook/play.py

@@ -44,7 +44,7 @@ class Play(object):
        'hosts', 'name', 'vars', 'vars_prompt', 'vars_files',
        'tasks', 'handlers', 'remote_user', 'user', 'port', 'include', 'accelerate', 'accelerate_port', 'accelerate_ipv6',
        'sudo', 'sudo_user', 'connection', 'tags', 'gather_facts', 'serial',
-       'any_errors_fatal', 'roles', 'pre_tasks', 'post_tasks', 'max_fail_percentage',
+       'any_errors_fatal', 'roles', 'role_names', 'pre_tasks', 'post_tasks', 'max_fail_percentage',
        'su', 'su_user', 'vault_password'
     ]