Added move_group capability for clearing octomap.

In addition to the capability and service, this adds a button to the motion planning rviz plugin as well.

This requires users to add the capability to their move_group.launch files before it will work.

I don't know much about the interactions between move_group components, so if this is likely to cause trouble please speak up.  I have tested this with a single camera writing to the octomap and it worked for me.