Fix re-docking problems, add tweakable

Fix a problem where the mass driver would not find its attached
part when the docking node was disconnected and then
re-connected.  This is done by removing any dependency on
the docking port module, and instead looking at all direct
children and parents, and comparing the vector between the
parts' positions with the direction that the mass driver is
facing.

Add a tweakable to control the power output of the mass driver.