qa/workunits/rest/test.py: make osd create test idempotent

Avoid possibility that we create multiple OSDs do to retries by passing in
the optional uuid arg.  (A stray osd id will make the osd tell tests a
few lines down fail.)

Fixes: #8728
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2061 from ceph/wip-8728

qa/workunits/rest/test.py: make osd create test idempotent

Reviewed-by: Dan Mick <dan.mick@inktank.com>