qa: use small default pg count for CephFS pools

qa: use small default pg count for CephFS pools

The pg count needs to be a power-of-two since
dff5697464edb9931d5dfa08cd4a30f85c1f237e.

Also, mon_pg_warn_min_per_osd is disabled by default now (or set to a
low value in vstart/testing) so there's no need to base the pg count on
this value.

Ideally someday we can remove this so that the default cluster value is
used but we need to keep this for deployments of older versions of Ceph.

Fixes: https://tracker.ceph.com/issues/42228
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit fc88e6c6c55402120a432ea47f05f321ba4c9bb1)

Conflicts:
	qa/tasks/cephfs/filesystem.py: this commit was orignally
backported by #34055, but it failed to cherry-pick all necessary
bits. in this change, the missing bit is picked up.