ceph: retry new-style mon command if we get EINVAL from the old-style command

We can race with a mon upgrade:

- get command descriptions, get EINVAL
- mons upgrade, new quorum
- send old-style command
- get EINVAL

In this case, we should try one last time to get the command descriptions.

Fixes: #5788
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>