Use the correct api to get the volume object.
Use the correct api to get the volume object.

It must be volume not nova. But nova is used for the attachment.

Signed-off-by: Angus Salkeld <86b65304d27d8de73dd7d624c33df7e088f8d94b@redhat.com>

NeCTAR-RC/heat,jasondunsmore/heat,bbandaru/heat-cfntools,citrix-openstack-build/heat,ntt-sic/heat,cwolferh/heat-scratch,rdo-management/heat,noironetworks/heat,steveb/heat-cfntools,dims/heat,rickerc/heat_audit,dragorosson/heat,openstack/heat,dragorosson/heat,rdo-management/heat,citrix-openstack-build/heat,varunarya10/heat,gonzolino/heat,redhat-openstack/heat,srznew/heat,redhat-openstack/heat,cwolferh/heat-scratch,pshchelo/heat,steveb/heat,steveb/heat,JioCloud/heat,sdake/heat-jeos,pratikmallya/heat,takeshineshiro/heat,miguelgrinberg/heat,cryptickp/heat,varunarya10/heat,openstack/heat-cfntools,srznew/heat,ntt-sic/heat,Triv90/Heat,rh-s/heat,Triv90/Heat,steveb/heat-cfntools,takeshineshiro/heat,rickerc/heat_audit,jasondunsmore/heat,JioCloud/heat,pshchelo/heat,Triv90/Heat,cryptickp/heat,noironetworks/heat,miguelgrinberg/heat,gonzolino/heat,maestro-hybrid-cloud/heat,dims/heat,NeCTAR-RC/heat,rh-s/heat,maestro-hybrid-cloud/heat,pratikmallya/heat,openstack/heat