mgr: simply exit on SIGINT or SIGTERM

mgr: simply exit on SIGINT or SIGTERM

The python modules don't reliable shut down.  We've fixed a few shutdown
bugs chasing https://tracker.ceph.com/issues/42744.

This was compounded by https://tracker.ceph.com/issues/42981, which seems
to suggest something in the python module teardown is screwing with the
signal handlers.

Now that df507cde8d71063d5873a42f668156e4c32e86f9 is there, the mgr will
blacklist all rados clients the mgr instantiates, which means we can just
exit immediately and let the blacklisting clean things up.

Works-around: https://tracker.ceph.com/issues/42981
Fixes: https://tracker.ceph.com/issues/42744
Signed-off-by: Sage Weil <sage@redhat.com>