Make repository selection configurable on RedHat

Our centos boxes are configured with multiple repositories
that contain the nginx package, and this role was previously
picking the wrong one.

This change aggressively prunes the enabled repo list to
minimize such potential conflicts, and adds configurability
just in case there's a name mismatch.
Merge pull request #7 from sethp-jive/wrong-repo-bug

Make repository selection configurable on RedHat