ceph-create-keys: gracefully handle no data from admin socket

Old ceph-mon (prior to 393c9372f82ef37fc6497dd46fc453507a463d42) would
return an empty string and success if the command was not registered yet.
Gracefully handle that case by retrying.

If we still fail to parse, exit entirely with EINVAL.

Fixes: #4952
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@intank.com>