Check if yum is really present on a 'redhat' machine. Only if it is,
enable the Yum bucket. The same for DNF since older versions of CentOS
and Fedora do not ship DNF.
Doing this is kinda irresponsible because we don't have any unit testing
in place but I've had enough of copy & pasting things around. Although it
doesn't make the codebase GREAT, I believe it'll reach a _nice_ status :)
Thanks to those changes I realized that skipping configuration of
buckets that have already been configured was easier than I thought and
should be enough to close GH-85 \o/
