nodes performance tweaks

simplejson is much faster than std lib json.  the pulp.server.compat attempts to import simplejson so better to use import json from there.

shutl.rmtree() is REALLY slow.  Removing a published RHEL 6 tree takes ~10 seconds using rmtree() and ~15 ms. using 'rm -rf'.  Not as portable but we can deal with that if we have to.  But for new, this really improves nodes publishing performance.