Add support for an unmanaged repository resource

This PR adds a `node['repose']['repo']['managed']` attribute to flag on or off the Repose repository. In cases where the repository is already configured or is managed by a different agent it would be desirable to be able to turn off the corresponding resource here. This is similar to repository definitions in the https://github.com/opscode-cookbooks/yum-epel and https://github.com/opscode-cookbooks/yum-centos.