Remove image caching from 'prepare'

Today we try to cache images when 'prepare' is called. It turns out
this doesn't quite match Ironic's model, where 'prepare' is more about
managing the local "deployment environment" which we don't really use.

Eventually we'll want to implement image caching as a vendor passthru
method.
Merge pull request #14 from rackerlabs/no_prepare

Remove image caching from 'prepare'