Simulator: use env var PX4_SIM_SPEED_FACTOR
Simulator: use env var PX4_SIM_SPEED_FACTOR

This changes setting the simulation speed to the env variable
PX4_SIM_SPEED_FACTOR instead of the cli arg "-f".
The cli interface is still possible but shows a deprecation warning.

The reason for this change is to be consistent with the Gazebo
interface.

PX4/jMAVSim,PX4/jMAVSim,PX4/jMAVSim