Correct image's random name

In image.base.create_image, random name is set to "-instance"
by mistake.
Besides, in _create_standard_image, random name definition is
unnecessary for there already random name exists in base.create_image.

Change-Id: I4826e4117b1a54077ca9aa2ec8690c104dc7e8e8
diff --git a/tempest/api/image/base.py b/tempest/api/image/base.py
index 26b88b0..1cc3fa2 100644
--- a/tempest/api/image/base.py
+++ b/tempest/api/image/base.py
@@ -60,7 +60,7 @@
         """Wrapper that returns a test image."""
 
         if 'name' not in kwargs:
-            name = data_utils.rand_name(cls.__name__ + "-instance")
+            name = data_utils.rand_name(cls.__name__ + "-image")
             kwargs['name'] = name
 
         params = cls._get_create_params(**kwargs)
diff --git a/tempest/api/image/v2/test_images.py b/tempest/api/image/v2/test_images.py
index aff8a78..6f8d239 100644
--- a/tempest/api/image/v2/test_images.py
+++ b/tempest/api/image/v2/test_images.py
@@ -157,9 +157,7 @@
         """
         size = random.randint(1024, 4096)
         image_file = six.BytesIO(data_utils.random_bytes(size))
-        name = data_utils.rand_name('image')
-        image = cls.create_image(name=name,
-                                 container_format=container_format,
+        image = cls.create_image(container_format=container_format,
                                  disk_format=disk_format,
                                  visibility='private')
         cls.client.store_image_file(image['id'], data=image_file)