osd/scheduler: Reset ephemeral changes to mClock built-in profile

osd/scheduler: Reset ephemeral changes to mClock built-in profile

This is a follow-up to PR: https://github.com/ceph/ceph/pull/48703.
This commit also considers changes made ephemerally using either the
'daemon' or the 'tell' interfaces to override the built-in mClock
QoS parameters. In such a scenario, the ephemeral changes are removed
using the rm_val() method exposed by the config subsytem and logging
this information.

Other changes:

1. Add a standalone test to exercise the fix.
2. Add documentation note on the outcome of the attempt to modify
   built-in profile defaults.

Fixes: https://tracker.ceph.com/issues/61155
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>