Use requests-mock for the volume backup tests
Use requests-mock for the volume backup tests

Change-Id: I88d7351a2fc147224542898d08e2a769a62577a0
Signed-off-by: Rosario Di Somma <73b2fe5f91895aea2b4d0e8942a5edf9f18fa897@gmail.com>

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