add respawn parameter to rviz launcher

In order to avoid having to manually restart rviz on startup failures
caused by video hardware problems, the most obvious solution is adding
the respawn property to the launcher. ctrl+c stops rviz when needed.
simple naming change for respecting conventions
set gravity and selfcollision to true in most of the model links

These changes only affect Gazebo as these are properties of the
<gazebo> tag. selfcollision should avoid the robot collapsing into
itself.