XXX-Tentative slapos/test: Always shall full diff for JSON schema problems

Without full diff it's often impossible to understand the problem of JSON
schema introduced by the developer.

slapos/test/test_json_schema.py

@@ -115,6 +115,8 @@ def generateJSONSchemaTest():
 
 
 class TestJSONSchemaValidation(unittest.TestCase):
+  # XXX: Tentative - show full difference to understand problems
+  maxDiff = None
   pass
 
 generateSoftwareCfgTest()