Use service role also in test_assisted_volume_snapshots

Assisted volume snapshots API is service-to-service API
and nova is changing its default permission to 'service'
role
- https://review.opendev.org/c/openstack/nova/+/892635

To pass test on new defaults as well as old defaults
testing jobs, we need to use service role also in
the Tempest creds. Once nova change is merged then we
can use only service role to access this API.

Needed-By: https://review.opendev.org/c/openstack/nova/+/892635
Change-Id: I2c4368b8e2378842919fd16fa6f07505605f283a
1 file changed