Use requests-mock for the list/add/remove volume types tests
Use requests-mock for the list/add/remove volume types tests

Change-Id: I03aac75375b0f34678f8dc0b953fb2a0cd797150
Signed-off-by: Rosario Di Somma <73b2fe5f91895aea2b4d0e8942a5edf9f18fa897@gmail.com>

dtroyer/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack-infra/shade,openstack-infra/shade