Use F18 disto boto version

Pinning to boto 2.5.2 doesn't work for F18 guests, because it doesn't
work properly with cloud-init 0.7 (ec2 metadata retrieval fails, bug
1158906)

So revert to the Fedora packaged boto, which doesn't work with
heat-cfntools yet (ref bug 1122472), but it does allow intital heat
customization via cloud-init (and cfn-init using local metadata
supplied via nova userdata) to work.