Rename image_client to images_client

Module names of service clients should be <resource name>s_client.py,
and class names also should be <resouce name>sClient.
So this patch renames image_client for matching to the naming rule.

Partially implements blueprint consistent-service-method-names

Change-Id: Id9d8488251d7b1a4642eab08270b787aa6c74050
diff --git a/tempest/cmd/javelin.py b/tempest/cmd/javelin.py
index b126ef6..c5e4a7e 100755
--- a/tempest/cmd/javelin.py
+++ b/tempest/cmd/javelin.py
@@ -128,7 +128,7 @@
 from tempest.services.compute.json import security_group_rules_client
 from tempest.services.compute.json import servers_client
 from tempest.services.identity.v2.json import identity_client
-from tempest.services.image.v2.json import image_client
+from tempest.services.image.v2.json import images_client
 from tempest.services.network.json import network_client
 from tempest.services.network.json import subnets_client
 from tempest.services.object_storage import container_client
@@ -213,7 +213,7 @@
                                                   **object_storage_params)
         self.containers = container_client.ContainerClient(
             _auth, **object_storage_params)
-        self.images = image_client.ImageClientV2(
+        self.images = images_client.ImagesClientV2(
             _auth,
             CONF.image.catalog_type,
             CONF.image.region or CONF.identity.region,