This allows for more config flexibility - e.g. running multiple
backends with a common frontend.

Note this is a building block for future work on letsencrypt
validator (which should offer backend and share frontend with
any service running off 80/443 - which would be only horizon
in the current default config), as well as any work towards
single port (that is single frontend) and multiple services
anchored at paths of it (which is the new recommended default).

Change-Id: Ie088fcf575e4b5e8775f1f89dd705a275725e26d
Partially-Implements: blueprint letsencrypt-https