Support SSL redirect

If enforce_ssl attribute is true, then the nginx proxy will redirect
HTTP to HTTPS. By default there are no SSL configuration options set
with this as we're using AWS ELB to terminate HTTPS. Adding the ssl
certificate and key, plus additional options was added to lay the
groundwork for potential expansion on this, but it needs to be tested
(which includes building a self-signed certificate for the repository
to do the testing).

Background on this configuration method:

http://frankmitchell.org/2013/05/https-elb/