Merge pull request #1 from larsks/master

Make test_rdo_upgrade run from a minimal CentOS install (and fix some packstack annoyances)
Support multinode installs and upgrade packstack configs

This adds a post-packstack config phase to hook in local changes
to facilitate testing a multinode install. It also does a couple
of things that packstack doesn't currently do, like upgrading its
own config and upgrading the packages of other nodes.