Change volume manager to volume type, unset property for type
Change volume manager to volume type, unset property for type

In the unset method in volume_type, it was calling the volume
manager, instead of the volume_type.

Bug: 1203561
Change-Id: Iea1a9214db90f15815a456955040c0c5a795ff3d

citrix-openstack-build/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient,metacloud/python-openstackclient,BjoernT/python-openstackclient,varunarya10/python-openstackclient,derekchiang/python-openstackclient,metacloud/python-openstackclient,BjoernT/python-openstackclient,openstack/python-openstackclient,citrix-openstack-build/python-openstackclient,dtroyer/python-openstackclient,derekchiang/python-openstackclient,varunarya10/python-openstackclient,redhat-openstack/python-openstackclient,openstack/python-openstackclient