Add global exclusions to dib-lint

This will be useful for adopting dib-lint in other projects, in
particular tripleo-image-elements.  It allows some dib-lint rules
to be used even if the project can't pass all of the checks.  The
failing checks can then be fixed one at a time and removed from the
exclusions list.

For consistency, this change reads the new exclusions from tox.ini
in the same way as flake8.  To use the exclusion mechanism, the
the following section can be added to tox.ini:

  [dib-lint]
  ignore = sete setu setpipefail

Change-Id: I6d8024e7613890e935ceb5e50d0d507bd554d8dd
Merge "Add global exclusions to dib-lint"