Correct boundary value test in ImagesOneServerNegativeTestJSON

The max character length of name is 255 in create_image, and
we should use the length of 256 for the negative boundary value
test.
Besides, testcase name test_create_image_specify_name_over_256_chars
is not correct, in which 256 should be 255.

see http://developer.openstack.org/api-ref/image/v2/index.html?expanded=create-an-image-detail#create-an-image

Change-Id: I924ae3a386f9609a2ce3f6fafe368fa7290435d1
diff --git a/tempest/api/compute/images/test_images_oneserver_negative.py b/tempest/api/compute/images/test_images_oneserver_negative.py
index d9b80e1..342b8fc 100644
--- a/tempest/api/compute/images/test_images_oneserver_negative.py
+++ b/tempest/api/compute/images/test_images_oneserver_negative.py
@@ -98,9 +98,9 @@
     @test.attr(type=['negative'])
     @test.idempotent_id('3d24d11f-5366-4536-bd28-cff32b748eca')
     def test_create_image_specify_metadata_over_limits(self):
-        # Return an error when creating image with meta data over 256 chars
+        # Return an error when creating image with meta data over 255 chars
         snapshot_name = data_utils.rand_name('test-snap')
-        meta = {'a' * 260: 'b' * 260}
+        meta = {'a' * 256: 'b' * 256}
         self.assertRaises(lib_exc.BadRequest, self.client.create_image,
                           self.server_id, name=snapshot_name, metadata=meta)
 
@@ -123,10 +123,10 @@
 
     @test.attr(type=['negative'])
     @test.idempotent_id('084f0cbc-500a-4963-8a4e-312905862581')
-    def test_create_image_specify_name_over_256_chars(self):
-        # Return an error if snapshot name over 256 characters is passed
+    def test_create_image_specify_name_over_character_limit(self):
+        # Return an error if snapshot name over 255 characters is passed
 
-        snapshot_name = data_utils.rand_name('a' * 260)
+        snapshot_name = ('a' * 256)
         self.assertRaises(lib_exc.BadRequest, self.client.create_image,
                           self.server_id, name=snapshot_name)