Add --legacy and --projects options to the owners CLI, which make it
possible to specify local alternatives for the legacy.yaml and
projects.yaml files. This enables easily running the tool for
unofficial/non-OpenStack elections which use the same Gerrit and
similar rules for determining their electorates other than the
actual repository lists.
Change-Id: I45d6c4d4fd454ae426883006db1e58dbd373bbdf