With polkit version 0.106, .rules files are used instead of .pkla files, see http://davidz25.blogspot.com/2012/06/authorization-rules-in-polkit.html for more information. Therefore, please port the file /etc/polkit-1/localauthority/50-local.d/50-nova.pkla to use the new format. If you have any questions, I'm subscribed to this bug to help. Thanks.
*** Bug 858311 has been marked as a duplicate of this bug. ***
http://pkgs.fedoraproject.org/cgit/openstack-nova.git/commit/?id=6c35007725038049d0331261cee45a02dc68ca2f
openstack-nova-2012.2-1.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/openstack-nova-2012.2-1.fc18
Package openstack-nova-2012.2-1.fc18: * should fix your issue, * was pushed to the Fedora 18 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing openstack-nova-2012.2-1.fc18' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2012-15033/openstack-nova-2012.2-1.fc18 then log in and leave karma (feedback).
openstack-nova-2012.2-1.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.