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.
don't migrate; use template subdir
install own /etc/nginx/nginx.conf

One comes with the standard installation, overwrite with our own,
provided in {{secure_dir}}/files.
make nginx.conf overriding optional

If the nginx play is called with the nginx_conf varialble, then expect
to find in {{secure_dir}}/files/nginx.conf and copy it over.  If not set
then the default one already there will be sufficient.
carnegie-app: adding ability to migrate back in

They have their own LMS DB.