Ensure that readonly result members are serialized

* fix for breaking metadata change in various Azure Python SDK bits; some members were marked `readonly` for validation, which the default msrest serializer ignores. Added `keep_readonly` flag to serializer call to ensure they're preserved.

lib/ansible/module_utils/azure_rm_common.py

@@ -528,7 +528,7 @@ class AzureRMModuleBase(object):
             self.log("dependencies: ")
             self.log(str(dependencies))
         serializer = Serializer(classes=dependencies)
-        return serializer.body(obj, class_name)
+        return serializer.body(obj, class_name, keep_readonly=True)
 
     def get_poller_result(self, poller, wait=5):
         '''