Add variant keyword to load_robot_description

[stephane-caron]

Some descriptions have variants (like the "left" and "right" versions of the Allegro hand) that are currently only available when importing submodules. We could add a variant keyword argument to load_robot_description so that users can access it from there as well.

[kevinzakka]

Also useful for loading end-effector-less versions of a robot (e.g. Panda, xArm7) which ship with an end-effector.