mgr/balancer: tolerate pgs outside of target weight map

mgr/balancer: tolerate pgs outside of target weight map

We build a target weight map based on the primary crush weights, and
ignore weights that are 0.  However, it's possible that existing PGs are
on other OSDs that have weight 0 because the weight-set weight is >0.
That leads to a KeyError exception when we

                     pgs_by_osd[osd] += 1

and the key isn't present.  Fix by simply populating those keys as we
encounter OSDs.  Drop the old initialization loop.  The net of this is
we may have OSDs outside of target_by_root (won't matter, as far as I can
tell) and we won't have keys for osds with weight 0 (also won't matter,
as far as I can tell).

Fixes: https://tracker.ceph.com/issues/42721
Signed-off-by: Sage Weil <sage@redhat.com>