mgr/PyModule: correctly remove config options

mgr/PyModule: correctly remove config options

Previously, incorrect parameters were being passed to "config rm",
causing it to do nothing.  This commit also ensures the correct
error message is shown for both the set and remove failure cases.
I've also moved the update of the in-memory config map to *after*
the value is persisted, to ensure the config map actually reflects
what's stored.

Fixes: https://tracker.ceph.com/issues/42958
Signed-off-by: Tim Serong <tserong@suse.com>
(cherry picked from commit 0520ff571cfb480872c8bd429c94bd4ce15eeb40)