linters: Take a guess at the roles path
linters: Take a guess at the roles path

It's probably reasonable to assume that for interactive use, the
required roles for linting in zuul-jobs and openstack-zuul-jobs are in
sibling directories of project-config.  Set this as the default
environment value.

Change-Id: I388fdae998ac3cb87c30616ed761c888e1e429f0

openstack-infra/project-config,openstack-infra/project-config