In case any one runs into this, there is a bug in the Precise upstart package that causes a reboot after installing essex to result in a kernel panic. I don't know exactly what in the openstack install process triggers it but there is a PPA mentioned in comment #11 of this bug ticket that fixes the problem after upgrading the upstart package: https://bugs.launchpad.net/ubuntu/+source/upstart/+bug/935585 -David