(work-in-progress) Add a bootstrap element:

The boostrap element produces an image which,
when booted, contains a minimal running openstack,
for the purpose of bootstrapping a larger deployment.

What it does now:
  - runs nova,glance,keystone,cinder,quantum
  - adds a minimal set of users/roles/etc.
  - runs nova-compute with the libvirt driver.

TODO:
  - reconfigure and test with baremetal driver
  - break services into component elements
  - touch a file or otherwise indicate that
     first-boot scripts have completed successfully
  - documentation.

Change-Id: Ieeba863ab7304f96df77afa95ea54658c877a730