Verify the DHCHAP and PSK values in the gateway

[gbregman]

When we use DHCHAP or PSK keys, we pass the values to the SPDK which verifies the content and fail the operation in case the key is invalid. The problem is we don't get a clear error code. SPDK just flag all error with the same generic code. In order to give a more readable error to the user we should add key verification code to the gateway. This will allow us to tell the user the exact problem. As the key spec is fixed and can't be changed we can duplicate the validation logic of SPDK in the gateway.