Autobackend Generation (v2)

---reworking a little bit more to be flexible in terms of required tags---
This feature affects the haproxy_autoscale module only and is backwards-compatible with previous templates and invocations of update-haproxy. Tests currently run system haproxy, but could be pointed at the local haproxy bundled if you wish.

Uncomment tests/test_haproxy_template.py#50 and comment out #51 if you want to inspect the configs generated by the tests.