Feature/upgrade to 0912

This PR fixed some basic issues for OpenCDA to support CARLA 0.9.12.

CARLA_VERSION environment variable now needs to be set to run setup.sh to install the correct api version.
opencda.py now also requires users to input their CARLA version to select the correct api.
Some changes on the default testing scenario configuration file to meet the new characteristics of the vehicle dynamics.
Corresponding documentation is updated.
All default testing scenarios are tested under 0.9.12, and they work fine.

ucla-mobility / OpenCDA