EUCA-6888: Imaging service update
  -- change how imaging worker VMs are deployed. Previously, the property
     'imaging.imaging_service_enabled' was used to trigger launching VMs.
     Instead the VMs are launched when the import-vol/instance api is called
     and no existing VMs are found.
     We still has the property 'imaging.imaging_worker_enabled', which is set
     'true' by default. By setting it 'false', admins can destroy resources used for workers.
     When the api is called later, the worker will be deployed again.
Merge branch 'testing' of git.eucalyptus-systems.com:eucalyptus into testing