Implement NetconfMessage.toString()

Rather than explicitly converting the internal document intoa string
before logging it, teach NetconfMessage how to do toString(). This way
we do not have to check Logger configuration and pass NetconfMessages
directly. The logger will cann toString() as needed.

Change-Id: I07a18f32a1500784b81f52f0412acd73123a161e
Signed-off-by: Robert Varga <rovarga@cisco.com>