Option to disable rendering of vehicles

[kfarr]

For loud bicycle game we need ability to render a street without placing vehicles in the z center of a street segment

Options:

1. flag on street component like showVehicles similar to showGround or showStriping with default = true. When set to false the vehicles are not placed in the scene.
2. streetmix json "variant" at the segment level that can control vehicle presence on a segment by segment basis

I suggest 1 as a starting point