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>
(cherry picked from commit bb3e1c92b6682ed39968dc5085b69c117f43cbb0)