Adjust scripts to upgrade to Cloud 4

* Handle possible absence of barclamp-pacemaker on a non-HA setup
* Write helper-neutron-migration to do the database migrations for neutron
* Rename reboot-node to prepare-and-reboot-nodes and extend it
@vuntz: Yeah, we do the migration in the init script. But for the 3->4 upgrade the migrations must run before any config changes happen. Especially the addtion of the "lbaas" service plugin in Cloud 4 breaks the migration if done after the config files are updated. :/