Clean up Berkshelf chef-solo support.

* Instead of creating package.tar.gz in the middle of our actual
  cookbooks, create it in /var/cache/crowbar/cookbooks.
* Don't try to pull in the berkshelf gem into rails just so we can run
  it in a subshell.  Instead, arrange for the subshell it should run in
  to be outside of the bundler environment.
* Have chef-solo die horribly if we don't have a package.tar.gz to copy
  over.
Merge pull request #113 from VictorLowther/fix-berkshelf-solo-support

=2 Clean up Berkshelf chef-solo support.