Create a new config setting to allow zuul executors to be grouped into
zones. By default, this setting is disabled (set to None), to keep
backwards compat.
Story: 2001125
Task: 4817
Change-Id: I345ee2d0c004afa68858eb195189b56de3d41e97
Signed-off-by: Paul Belanger <pabelanger@redhat.com>