MT-173: speed up first centos_base run

This is a very minor fix to the comment line of the repo files generated by the version of the opscode yum cookbook that we're using internally.  The change is consistent with the current version of the opscode recipe: https://github.com/opscode-cookbooks/yum/blob/master/templates/default/repo.erb

The idea is to eliminate the hostname from the yum repo files so that we can generate the repo files during our image build process.  Generating the repo files during the image build process eliminates the need for running yum makecache or yum clean all when centos_base runs for the first time after boot.

I considered upgrading this cookbook to the current version, but would prefer to make a surgical change to a comment rather than dealing with all of the other internal dependencies that we have on the yum cookbook.