trivial - allow disabling repo creation prior to install, just use system repos

In cases - especially the Enterprise - where repositories are either pre-defined or merged in another repo (Think:Spacewalk channels), adding a remote software repo and having the managed node connect to the world for its updates is somewhere between a bad idea and a really bad idea.

In those enterprise cases only, it could make sense to NOT create the repo profile for yum (and apt) and just use what's already provided by the enterprise admin.

This simple patch adds an attribute and code to disable the repo generation only, defaulting to classic, regular 'please create repos' behaviour.

Tested with Centos7 only.