Adds support for do_not_generate_frontend key in haproxy hash.

I've found I've wanted to create multiple backends which are referenced by a single frontend, but with more than one frontend (listening on different interfaces/ports), which means the shared frontend option was unsuitable. Being able to omit the unnecessary frontends by a single "do_not_generate_frontend": true" line in the haproxy block for a service solves this problem neatly.