This change simplifies the path creation logic to avoid processing
user defined variables such as job name and pipeline name, which
might cause the log url to exceed the database storage presently
fixed at 255 char.
Add warning in the job's header when the url is over 255 characters,
explaining that Zuul won't report the job properly in its database;
but the job can still run.
Change-Id: I34fb5662a2f958c55f60458ce107bad2a73b9c96
2f7ffba18d