cephadm: gracefully shutdown the agent prior to removing

cephadm: gracefully shutdown the agent prior to removing

When the agent is removed, the daemon is abruptly stopped.
Since the node-proxy logic runs from within the cephadm agent,
it leaves an active RedFish session. The idea is to gracefully
shutdown the agent so node-proxy can catch that event and make sure
it closes the current active RedFish session prior to shutting down.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit 79bfe642001a7f9e1da28f987d1edb45174f6e86)