Apply a naming rule of GET to volume client

[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to volume client.

Partially implements blueprint consistent-service-method-names

Change-Id: I843dd43293f40bf673a033b1dbf24e035bc8f0c3
diff --git a/tempest/api/volume/test_volumes_extend.py b/tempest/api/volume/test_volumes_extend.py
index 35c12bc..38bb748 100644
--- a/tempest/api/volume/test_volumes_extend.py
+++ b/tempest/api/volume/test_volumes_extend.py
@@ -35,7 +35,7 @@
         extend_size = int(self.volume['size']) + 1
         self.client.extend_volume(self.volume['id'], extend_size)
         self.client.wait_for_volume_status(self.volume['id'], 'available')
-        volume = self.client.get_volume(self.volume['id'])
+        volume = self.client.show_volume(self.volume['id'])
         self.assertEqual(int(volume['size']), extend_size)