mon/AuthMonitor: make initial auth include rotating keys

This closes a very narrow race during mon creation where there are no
service keys.

Fixes: #5427
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #374 from ceph/wip-5427

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>