Added parameterless BusAttachment.Disconnect method

Just like the BusAttachment.Connect() method allows the user to connect the
BusAttachment without knowing the connectSpec the BusAttachment.Disconnect()
method allows Disconnect to be called without knowing the connectSpec.

Internally Disconnect calls BusAttachment.Disconnect(GetConnectSpec())

Change-Id: I99f6892f6a4c5a43b2536fb4167994a9e872ff2d