[CHEF-4029] ensure the `enable_non_ssl` nginx attribute works

Currently trying to enable non-ssl mode has no effect. This commit
ensures we render a both an HTTP and HTTPS version of the Chef API lb
config. This behavior follows what we do in Private Chef also.
[CHEF-4029] don't override user provided nginx url

If the user did not provide a value for the nginx `url` we will
construct one, taking the value passed into `ssl_port` into account.
Fix regression introduced with 9f9cc08aa1

The merge was being performed incorrectly on the variables attribute NOT
the hash going into that attribute.