rgw.py: change --secret-key to --secret

A 'user create' call was being passed to radosgw-admin
with '--secret-key' instead of the valid '--secret'
which was causing a random secret to be generated,
which was causing subsequent tests to fail.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Merge branch 'next' of https://github.com/ceph/teuthology into next