This allows us to start a zookeeper instance via docker as part of the
standard tox process, avoiding issues like those described in . It's
exposed as a separate target, to ensure users can use local
installations if necessary. This requires bumping the minimum version of
tox to 3.2 so we can make use of the 'requires' config option .
Signed-off-by: Stephen Finucane <email@example.com>