Changes

onomondo_eim_app: unify HTTP/HTTPs server startup

The REST API also makes use of the cowboy HTTP server in the same
way as the ESipa API does when running without SSL/TLS. Let's unify
the code in such a way that both HTTP servers call the same startup
helper functions.

Related: SYS#8100
Change-Id: Ie63a97b7ca6873d02866a11226733b37017504cd