rgw: use different keys for different system users

Only one of these was being used before, due to a bug in
teuthology. This is fixed, and trying to create a second user with the
same access key fails.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
rgw: add data sync test

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>