osd: make open classes on start optional

Some might want the old behavior.

Signed-off-by: Sage Weil <sage@inktank.com>
upstart: stop ceph-create-keys when the monitor stops

This avoids lingering ceph-create-keys tasks.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>