Detach Volumes from an Instance in parallel
Detach Volumes from an Instance in parallel

Use a PollingTaskGroup to ensure that if there are multiple Volumes
attached to an Instance they are detached in parallel rather than serially.

Change-Id: Ice399ae495af75558f2f8b1b1dc330d0b556a269

JioCloud/heat,cwolferh/heat-scratch,rickerc/heat_audit,ntt-sic/heat,NeCTAR-RC/heat,miguelgrinberg/heat,pshchelo/heat,dims/heat,takeshineshiro/heat,takeshineshiro/heat,miguelgrinberg/heat,NeCTAR-RC/heat,dragorosson/heat,openstack/heat,rdo-management/heat,redhat-openstack/heat,gonzolino/heat,jasondunsmore/heat,JioCloud/heat,rh-s/heat,maestro-hybrid-cloud/heat,pshchelo/heat,dims/heat,varunarya10/heat,srznew/heat,cryptickp/heat,redhat-openstack/heat,pratikmallya/heat,dragorosson/heat,pratikmallya/heat,openstack/heat,cwolferh/heat-scratch,steveb/heat,noironetworks/heat,cryptickp/heat,ntt-sic/heat,noironetworks/heat,citrix-openstack-build/heat,rickerc/heat_audit,rdo-management/heat,jasondunsmore/heat,maestro-hybrid-cloud/heat,citrix-openstack-build/heat,rh-s/heat,gonzolino/heat,steveb/heat,srznew/heat,varunarya10/heat