Update XML schema definition for launch files

[joshwapohlmann]

Problem

The only XML schema definition for launch files I could find was in the design docs.
But it is outdated, since e.g. it expects the <node> tag to have a executable attribute, but it is actually exec.
Now, I don't know about other inconsistencies, but I want to use this in my launch files. So what do do?

Proposed Solution

1. Update the schema definition
2. Host a copy on a public url as proposed in Provide roslaunch xml schema definition downloadable  launch#392

[ivanpauno]

Closing, as #286 was merged.
@joshwapohlmann If there's any other necessary update in the schema please let me know and I will help to fix it.

[ivanpauno]

Host a copy on a public url as proposed in ros2/launch#392

I will reply about that in #392.