Switch to use primary credential in volume backup tests

Not all the apis in tempest/api/volume/admin/test_volumes_backup.py
should use admin credential, some apis could switch to use primary
credential. See [1].

[1] https://github.com/openstack/cinder/blob/master/cinder/policies/backups.py

Change-Id: Ied77f68c982f3a8445956ad1a4ea47c3795d3294
1 file changed