Use show_volume_type to check the result of update_volume_type

In test_volume_type_update, after calling update_volume_type, it
only checks the response body of update_volume_type, and we should
call show_volume_type to check whether the volume type is really
updated.

Change-Id: I6b7a58a47bab22286e4ff783e9053def4f692ecc
1 file changed