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>
(cherry picked from commit e2528ae42c455c522154c9f68b5032a3362fca8e)
osd: don't assert if get_omap_iterator() returns NULL

Fixes: #4949
This can happen if the object does not exist and it's
a write operation. Just return -ENOENT.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 36ec6f9bce63641f4fc2e4ab04d03d3ec1638ea0)