pybind/mgr/autoscaler: don't scale pools with overlapping roots

pybind/mgr/autoscaler: don't scale pools with overlapping roots

In the previous version of get_subtree_resource_status() in
src/pybind/mgr/pg_autoscaler/module.py we ignore overlapping
pools which in some cases if combined with the new `scale-down`
algorithm in https://github.com/ceph/ceph/pull/38805 can cause
some pools to scale up/down to inapproriate amount of pgs.

Therefore, the PR identifies the overlapping roots and prevent the pools
with such roots from scaling. This only happens with `scale-down` profile
as we see no problem with the default `scale-up` profile.

Removed the variable `pool_root` since it is not used anywhere in
the code, it only gets assigned and reassigned

Also included a unit test test_overlapping_roots.py that tests the function
identify_subtrees_and_overlaps() as well as edited test_cal_final_pg_target.py
to account for pools that contain overlapping roots, therefore, those pools
are expected not to scale.

Signed-off-by: Kamoltat <ksirivad@redhat.com>