Handle alljoyn_busattachment_disconnect with a NULL connectSpec

Just like the alljoyn_busattachment_connect() method allows the user to connect the
BusAttachment without knowing the connectSpec the alljoyn_busattachment_disconnect()
method allows disconnect to be called without knowing the connectSpec.

Internally Disconnect calls BusAttachment.Disconnect(GetConnectSpec())

Change-Id: I2cb1435291497fb2fe29b2f10637c927e81288a1