Use tempest's ServiceClients rather than Manager

tempest.Manager has been deprecated for more than 4 years already
and will be removed by [1].
Tempest plugins are expected to consume
tempest.lib.services.clients.ServiceClients directly.

[1] https://review.opendev.org/c/openstack/tempest/+/767628

Change-Id: I870f3662679dddf8f29328b10c7fd9637dad9436
1 file changed