osd: Set initial mClock QoS params at CONF_DEFAULT level

osd: Set initial mClock QoS params at CONF_DEFAULT level

Create the initial mClock QoS params at CONF_DEFAULT level using
set_val_default(). This allows switching to a custom profile on a
running OSD and to make necessary changes to the desired QoS params.
Note that Switching to ‘custom’ profile and then subsequently changing
the QoS params using “config set osd.n …” will be at a higher level i.e.
at CONF_MON.

But When switching back to a built-in profile, the new values won’t take
effect since CONF_DEFAULT < CONF_MON. For the values to take effect, the
config keys created as part of the ‘custom’ profile must be removed from
the ConfigMonitor store after switching back to a built-in profile.

- Added a couple of standalone tests to exercise the scenario.
- Updated the mClock configuration document and the mClock internal
  documentation with a couple of typos relating to the best effort weights.
- Added new sections to the mClock configuration document outlining the
  steps to switch between the built-in and custom profile and vice-versa.

Fixes: https://tracker.ceph.com/issues/55153
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>