Grass too high should slow down the mower

[alexvaut]

When the grass is too high, the mower continues at the same speed, leaving the grass as is. Implementation: First we must allow the user to define the prefered speed and then, optionally, when the mower detects that the mower motor speed is too low then it will decrease the mower wheel speed during 1 minute or so. Everything should be configurable through env var.