Add switch to not manage SCL setup

[bodgit]

Currently this module tries to manage the various SCL packages, which creates a few problems:

1. Duplicate package resources for scl-utils with any other module that might also try to manage SCL.
2. The centos-release-scl package contains the default repository configuration which is not always accessible in some enterprise environments so it is desirable to override the repositories with an internal mirror.

I propose adding a simple manage_scl boolean which can default to true for the purposes of maintaining current behaviour, but wraps the centos-release-scl and scl-utils packages so setting it to false will take these out of the catalog.