protoproxy: call enable_ipv6_proxy in a sane way

As we need to define a variable influencing all protoproxy resources, I
created a protoproxy::params class that is then declared in the roles
that used dynamic lookup variables before (that does not work in
puppet3).

This design pattern should be used in general for this kind of situations.

Change-Id: I00d87dc1081a4c97ecca7d7a63e0358b638f4e67