(PUP-2944) Use osfamily to choose yum as a default

This commit changes the defaultfor from using the operatingsystem fact
to the osfamily fact. This is done so that we do not have to duplicate
effort of all the RedHat derivatives that use yum.

Signed-off-by: Charlie Sharpsteen <chuck@puppetlabs.com>
Merge pull request #2883 from Sharpie/pup-2944-yum-osfamily

(PUP-2944) Use osfamily to choose yum as a default