Allow for multiple frontends and backends

[widhalmt]

We need to have several frontends and backens running on a single HAProxy in a setup we run.

I'll gladly submit a PR for a proposed solution. @geerlingguy what do you think? Should I leve the current implementation intact and add variables like extra_frontends and extra_backends or rebuild the whole code so it works with multiple instances?

The former solution would help with backwards compatibility for configuration but split the same kind of configuration into two variables. The latter would make it harder to upgrade.

A possible third solution would be to implement new variables but have tasks that transform old configuration if it's present.

Just let me know what you prefer and I can build it into a PR. If I was too confusing or want to know more about my idea, just let me know, please.