Allow specifying unique-id-format and unique-id-header

[hany]

HAProxy has a very neat feature:

https://cbonte.github.io/haproxy-dconv/1.6/configuration.html#4.2-unique-id-format
https://cbonte.github.io/haproxy-dconv/1.6/configuration.html#4.2-unique-id-header

Not sure if it's currently possible to add these options. If not, it should be configurable.

[benwebber]

At the moment I want to avoid adding more discrete top-level configuration items since we're always playing catch-up with users' use-cases. I really think version 2 of this role requires less structured input.

It will, however, be possible to enter extra configuration lines once we merge #88.