Support system role directory

[elaske]

Currently this only looks at one place: <playbook>/roles. This should be more intelligent and also look at the entire roles directory list: http://docs.ansible.com/ansible/latest/user_guide/playbooks_reuse_roles.html#role-search-path (note, only one extra is required right now).

But as an extension, should probably look at the global ansible.cfg file to figure out which other places to look defined in roles_path.