This allows for more config flexibility - e.g. running multiple
backends with a common frontend.
It is not possible with the 'listen' approach (which enforces
frontend).
Additionally, it does not really make sense to support two ways
to do the exact same thing as the process is automated and
'listen' is really meant for humans not willing to write separate
sections.
Hence this deprecates 'listen' variant.

At the moment both templates work exactly the same.
The real flexibility comes in following patches.

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: I2362aaa3e8069fe146d42947b8dddf49376174b5
Partially-Implements: blueprint letsencrypt-https