[finishes #42401241] - when recreating a vm with persistent disk don't detatch the disks before shutdown

We don't want to yank the disks out from under running processes.   Best to shutdown the vm and assume that jobs have proper shutdown scripts
to sync data to disks.   Then we can recreate the vm and reattach the persistent disk to it.
[bosh_aws/openstack_cpi] pull registry initialization into its own method