[cinder][qa] Skipped the unstable test_volume_backup_create_get_detailed_list_restore_delete

Hello Cinder team, Test test_volume_backup_create_get_detailed_list_restore_delete is failing very frequently now a days. 19 fails in 24 hrs / 137 fails in 10 days [1] I have skipped this test temproarly[2]. mriedem has pointed out the failure on review and cinder channel also, please refer that for more details. Feel free to propose to unskip this test once the problem is fixed. [1] http://status.openstack.org/elastic-recheck/#1483434 [2] https://review.openstack.org/#/c/651660/ -gmann