Refactor random naming
Refactor random naming

'create_volume_type' method creates a very long and
unnecessary name, this patch do some clean code.

Change-Id: I45bc7043a513056740ec1beef147422ed9b035a8

masayukig/tempest,cisco-openstack/tempest,openstack/tempest,openstack/tempest,cisco-openstack/tempest,masayukig/tempest