module_utils.modules: expose module.params as a readonly dict

Use a MappingProxyType without backend to ensure the `module.params`
dict is readonly.

plugins/module_utils/modules.py

@@ -58,7 +58,7 @@
 import os
 import re
 import traceback
-
+from types import MappingProxyType
 
 try:
     from cStringIO import StringIO
@@ -120,6 +120,7 @@ def __init__(self, **kwargs):
             kwargs["argument_spec"] = argument_spec_full
 
         self._module = AnsibleAWSModule.default_settings["module_class"](**kwargs)
+        self._ro_params = MappingProxyType(self._module.params)
 
         if local_settings["check_boto3"]:
             if not HAS_BOTO3:
@@ -171,7 +172,7 @@ def __init__(self, **kwargs):
 
     @property
     def params(self):
-        return self._module.params
+        return self._ro_params
 
     def _get_resource_action_list(self):
         actions = []