Allow custom deploy.conf configurations for the oncue service component

This allows users to provide custom deploy configurations on top of the out-of-the-box `application.conf` provided by `oncue-service`.

Adding a `deploy.conf` in the `conf` directory (`.gitignore`d) allows you to add deployment specific changes without having to store the contents of `oncue-service`'s `application.conf` and overwrite it with your own settings.