Temporarily disable Cinder volumes backup tests

Temporarily disable Cinder volumes backup tests

From the time being, all OPNFV installers [1][2][3] don't support
Cinder volumes backup.

We will find a better way to handle the lack of OPNFV requiremements
(these tests pass succesfully in Orange Openlab).

At least, it will allow verifying tempest_full in OPNFV gates.

[1] Apex: https://artifacts.opnfv.org/logs/functest/lf-pod1/2018-10-25_02-17-25/tempest_full/tempest-report.html
[2] Compass: https://artifacts.opnfv.org/logs/functest/huawei-virtual9/2018-10-24_04-54-25/tempest_full/tempest-report.html
[3] Fuel: https://artifacts.opnfv.org/logs/functest/lf-pod2/gambia/2018-10-25_05-57-54/tempest_full/tempest-report.html

Change-Id: I319536070aa77a5dbab6a692a1d76049f75e758b
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 7cf46992c7f3e854b038f6f17a279d65108aa764)