crimson/osd: use stop_signal from seastar

crimson/osd: use stop_signal from seastar

and disable app_cfg.auto_handle_sigint_sigterm, otherwise app template
handles SIGINT and SIGTERM by itself, and calls app.stop(). but we don't
use this mechinary at all. we use seastar::defer() instead of
seastar::at_exit() for doing graceful shutdown and cleanup.

Signed-off-by: Kefu Chai <kchai@redhat.com>