Change terms

* 'start', 'work' -> 'base'
* 'final', 'trunk' -> 'target'
Rework directory structure

Create top-level directories for each release and put devstack and
the project repos under that.  data is still a top-level as it
will be processed for the upgrade.
General cleanup:

* Move *_RUN_EXERCISES after localrc
* Fix images cache dirs
* Save cinder database