template subdir; ownership on sites-avail/ file

To support variations on our local LMS sites files it is helpful to have
subdirectories where we can store variations.  At Stanford I'm using
this so I can store carnegie/lms.j2, a variation on our normal lms.j2.

To use you can call the role like this:
    - {'role': 'edxapp', 'openid_workaround': true, 'template_subdir': 'carnegie'}
This would be from your main playbook file, in my case
edx-west/carnegie-prod-app.yml.

While in here I noticed we weren't setting owner or perms on the
sites-available/XX files, fix that.

playbooks/roles/nginx/tasks/nginx_site.yml

 # Requires nginx package
 ---
 - name: nginx | Copying nginx config {{ site_name }}
-  template: src={{ item }} dest=/etc/nginx/sites-available/{{ site_name }}
+  template: src={{ item }} dest=/etc/nginx/sites-available/{{ site_name }} owner=root group=root mode=0600
   first_available_file:
+  - "{{ local_dir }}/nginx/templates/{{ template_subdir }}/{{ site_name }}.j2"
   - "{{ local_dir }}/nginx/templates/{{ site_name }}.j2"
   # seems like paths in first_available_file must be relative to the playbooks dir
   - "roles/nginx/templates/{{ site_name }}.j2"