Remove the duplicated error message from Enum

If Enum validation detects invalid parameter, wsme returns a respose
like the following message:

  {"debuginfo": null, "faultcode": "Client",
   "faultstring": "Invalid input for field/attribute param_enum.
                   Value: 'hourse'.
                   Invalid value (should be one of: dog, cat)"
  }

In faultstring, the first message shows a invalid request.
So it is not necessary to contain it in the third message.

Change-Id: Ie52660fcf704551ce082f7dca2fadedadb12562a
Merge "Remove the duplicated error message from Enum"