Define v1 images_client as library

Image v1 API's images_client has good interfaces now,
then this patch makes the client as a library.

Partially implements blueprint consistent-service-method-names

Change-Id: I8a0346e5526bfb83e54e0184bbc34ad5195203c3
diff --git a/tempest/clients.py b/tempest/clients.py
index b7bc4fa..ea8dd53 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -21,13 +21,13 @@
 from tempest import config
 from tempest import exceptions
 from tempest.lib.services import compute
+from tempest.lib.services import image
 from tempest.lib.services import network
 from tempest import manager
 from tempest.services import baremetal
 from tempest.services import data_processing
 from tempest.services import database
 from tempest.services import identity
-from tempest.services import image
 from tempest.services import object_storage
 from tempest.services import orchestration
 from tempest.services import volume