Expose drivetrain voltage limit.

The drivetrain control loop was assuming it could always use 12 volts.
That's not always a good idea (there are cases where we want to limit to
like 4 volts so we can tolerate stalling the drivetrain for extended
periods).  Make it configurable over the goal queue!

Change-Id: Ia13542d0ce0523dae31f30e10ae44c1872e06d1e
diff --git a/frc971/control_loops/drivetrain/ssdrivetrain.h b/frc971/control_loops/drivetrain/ssdrivetrain.h
index ce12b19..db255d7 100644
--- a/frc971/control_loops/drivetrain/ssdrivetrain.h
+++ b/frc971/control_loops/drivetrain/ssdrivetrain.h
@@ -48,6 +48,9 @@
 
   double last_gyro_to_wheel_offset_ = 0;
 
+  constexpr static double kMaxVoltage = 12.0;
+  double max_voltage_ = kMaxVoltage;
+
   // Reprsents +/- full power on each motor in U-space, aka the square from
   // (-12, -12) to (12, 12).
   const ::aos::controls::HVPolytope<2, 4, 4> U_poly_;