(RE-2158) Add build-tools param to rpmbuilder module

This commit adds a top level parameter to the rpmbuilder module to
enable the pl-build-tools repo, an internal repository to Puppet Labs.
It defaults to false to ensure backward compatibility. That parameter is
directly looked up in both the el7 and pl mock templates. This is to
avoid passing the parameter down through 5 levels of resource
indirection.
Add note explaining why skip_if_unavailable is a terrible idea
Remove puppet and rubygem-rgen from the epel el7 mock, they are now available at yum.pl.com
Merge pull request #69 from haus/ticket/master/re_2158_add_build_tools_repo

(RE-2158) Add build-tools param to rpmbuilder module