Add variable to demo.launch to control starting rviz for simpler test launch files.

[tylerjw]

@v4hn commented this here about the redundancy needed to build a test in moveit without launching rviz:

This is a lot of redundancy in the launch files and the only reason why you did not use the demo.launch is because it does not include a flag to disable rviz.

I'm fine with merging this patch as-is, but maybe we want to have such a flag in the demo.launch in the future so that tests can just include that one?

I suggest we add a use_rviz variable to demo.launch that defaults to true to simplify the launch files needed for using this for testing in moveit. This won't break existing use cases of demo.launch and then the functionality of it won't have to copy-pasted into launch files for tests.

[tylerjw]

In case anyone finds this, it was pointed out that these are generated files from the moveit setup assistant and that I should instead make a PR there and re-generated these files. Here is a link to my resulting PR: moveit/moveit#2019

[tylerjw]

This is finally becoming part of panda_moveit_config due to updating this package to match what the master branch on moveit generates.