Neutron should have better regex matching in zuul jobs

[Brian Haley]

Jobs defined in Neutron files in zuul.d sometimes have a large list of items in their 'irrelevant-files' sections, which recently got larger when we updated (and fixed bugs) moving to RE2.

There might be a better way to do this after discussions on the ML and IRC, for example here is a response from Clark Boylan:

"For Neutron it appears that the intention of those irrelevant files rules is to keep jobs from running when source code is not modified. I made a suggestion in IRC today that Neutron update their jobs to do something like:

  files:
    - ^.*\.py
    - zuul.d/project.yaml
  irrelevant-files:
    - ^neutron/tests/unit/.*$
    - ^neutron/tests/fullstack/.*$
    - other overmatch exclusion rules here

This matches the intended behavior of "only run this job when source code is changed" much better than managing a massive amount of irrelevant files rules. In the ironic case they decided they could just run bandit jobs more often. I think this is also a valid approach as we often try to over optimize these rules and confuse ourselves (the current situation is an excellent example of this)."

We should, as time permits, try to move to something like this, assuming it works as described.