Enforce boolean type for dup_ok

Stop "choices" from being interpreted as strings.

lib/ansible/modules/cloud/amazon/iam_cert.py

@@ -230,7 +230,7 @@ def main():
         new_name=dict(default=None, required=False),
         path=dict(default='/', required=False),
         new_path=dict(default=None, required=False),
-        dup_ok=dict(default=False, required=False, choices=[False, True])
+        dup_ok=dict(default=False, required=False, choices=[False, True], type='bool')
     )
     )