spawn_husky.launch: support argument to set urdf file and initial pose

[k-okada]

• passing parameters via environment is not useful, use arg of roslaunch
• to keep backworkd compatibility, we support previous way to set parmas using environment variables

[paulbovbel]

Happy to support both approaches. It's very useful for https://github.com/husky/husky_customization/blob/indigo-devel/husky_custom_gazebo/env-hooks/60.husky_custom_gazebo.sh

[k-okada]

ok, used optenv