switching to repo enable/disable on the yum command vs permanent

The previous bootstrap script would disable all repos and then install
various new repos.  What this meant was that if you ran the bootstrap
script more than once (say the 1st run errored for some reason) it would
put your system in a state where it couldn't install katello on the 2nd
run.  This update moves to setting the right repos during the install
command.
Merge pull request #6 from mccun934/rentrantbootsrap

switching to repo enable/disable on the yum command vs permanent