Use service role also in test_server_external_events

Server external events API is service-to-service API
and nova is changing its default permission to 'service'
role
- https://review.opendev.org/c/openstack/nova/+/892633

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/+/892633
Change-Id: Idff5879941ab1e11a527f2fd3c6577a51f41433c
1 file changed