Allowed planning scene monitor to ignore joint states that are not used

Baxter publishes two [joint states that are fixed](https://github.com/RethinkRobotics/baxter_common/issues/19) in its URDF. Because these variables do not exists, it throws an exception.

I also noted some potential optimizations that perhaps could be done - I haven't done them yet though because I'm not sure if I understand it fully.