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.
Pathing fixup

Set carnegie local relative to carnegie secure, so we have to get the
prefixes right once only.
Merge pull request #253 from edx/sef/nginx-long-sites-modes

nginx configuration for long site names