docker prepare script does not install docker before running docker command

[James Li]

In devstack log on gate:

2014-05-21 02:57:07.277 | ++ /opt/stack/new/solum/contrib/lp-cedarish/docker/prepare
2014-05-21 02:57:07.282 | Creating slugbuilder container
2014-05-21 02:57:07.286 | /opt/stack/new/solum/contrib/lp-cedarish/docker/prepare: line 15: docker: command not found
2014-05-21 02:57:07.291 | /opt/stack/new/solum/contrib/lp-cedarish/docker/prepare: line 16: docker: command not found
2014-05-21 02:57:07.295 | Creating slugrunner container
2014-05-21 02:57:07.300 | /opt/stack/new/solum/contrib/lp-cedarish/docker/prepare: line 23: docker: command not found
2014-05-21 02:57:07.305 | /opt/stack/new/solum/contrib/lp-cedarish/docker/prepare: line 24: docker: command not found
2014-05-21 02:57:07.310 | Preparing Buildpacks
2014-05-21 02:57:07.316 | /opt/solum/buildpacks ~/python-solumclient
2014-05-21 02:57:07.321 | Cloning into 'heroku-buildpack-java'...
2014-05-21 02:57:07.326 | Cloning into 'heroku-buildpack-ruby'...
2014-05-21 02:57:07.331 | Cloning into 'heroku-buildpack-python'...
2014-05-21 02:57:07.336 | Cloning into 'heroku-buildpack-nodejs'...
2014-05-21 02:57:07.340 | Cloning into 'heroku-buildpack-play'...
2014-05-21 02:57:07.345 | Cloning into 'heroku-buildpack-php'...
2014-05-21 02:57:07.350 | Cloning into 'heroku-buildpack-clojure'...
2014-05-21 02:57:07.354 | Cloning into 'heroku-buildpack-go'...
2014-05-21 02:57:07.359 | Cloning into 'heroku-buildpack-scala'...
2014-05-21 02:57:07.364 | Cloning into 'heroku-buildpack-dart'...
2014-05-21 02:57:07.369 | Cloning into 'heroku-buildpack-perl'...
2014-05-21 02:57:07.373 | Cloning into 'heroku-buildpack-mono'...

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/94661