Refactor 'detach vopt' to use detach_storage
Refactor 'detach vopt' to use detach_storage

pypowervm.tasks.scsi_mapper.detach_storage provides the function to
detach the storage element from VSCSI mappings.  This logic was
previously inlined in nova_powervm.virt.powervm.media.add_dlt_vopt_tasks
- this change set replaces the inline code with the pypowervm call.

Change-Id: Ica5096e4150d76c79c198073a3b96dab5789ab57

openstack/nova-powervm,stackforge/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm