mgr: add --max <n> to 'osd ok-to-stop' command

mgr: add --max <n> to 'osd ok-to-stop' command

Given and initial (set of) osd(s), if provide up to N OSDs that can be
stopped together without making PGs become unavailable.

This can be used to quickly identify large(r) batches of OSDs that can be
stopped together to (for example) upgrade.

Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit 722f57dee1306007c9059711a93b445bbd9079ec)

Conflicts:
        src/mgr/DaemonServer.cc
- in nautilus, cmd_getval() call requires g_ceph_context as first variable