Make Facter plugin puppet-output portable.

Switch from depending on the RPM library to a simpler (stupider)
method of detecting puppet. Just check if /usr/bin/puppet exists.

Closes #16
Funnel facter commands stderr into /dev/null

Previously, 'stderr' output was combined with stdout when calling
'commands.getstatusoutput'. This is unparsable and a problem on stock
Debian systems.

Closes #17
Merge pull request #18 from RHInception/facterpuppetportable

Facterpuppetportable