Define _set_image_clients
Define _set_image_clients

Align client setup for the image service in the client manager to
the other services. This will be useful for further refactor later.

Change-Id: I86651bcc535c00986ec181697bf64e43d5e94f54
Partially-implements: bp client-manager-refactor

cisco-openstack/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,vedujoshi/tempest,Juniper/tempest,sebrandon1/tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,sebrandon1/tempest,masayukig/tempest,openstack/tempest,Tesora/tesora-tempest,vedujoshi/tempest