extend doc jobs to support job_weight parameter

[tfoote]

The rosindex job needs the oversized memory machines. This works fine right now if the machine is mostly idle. But tonight I merged several packages during the rosindex run and I believe that it ran out of memory just before it finished.

I manually changed the doc_rosindex to use job_weight: 4 which will guarentee that the job has a dedicated executor

But that manual change won't persist. We'll need to add the parameter to the config here and to ros_buildfarm to know how to parse it and inject it.