qa: Reduce number of osd threads when using compression

qa: Reduce number of osd threads when using compression

Smithi nodes used by teuthology tests have 8 CPU cores and typically run
4 OSD processes. When bluestore software compression is enabled the size
of the OSD thread pool needs to be reduced to 2 threads per OSD because
these threads can easily use 100% of a core. This avoids excessive
amounts of context switches, which leads to OSD threads timing out,
which causes the OSD to drop heartbeat pings and for the monitor to
temporarily mark it down. In extreme cases this can lead to PGs getting
stuck in repeated loops of peering until the teuthology test times out.

Context switches happen oppurtunistically at the end of system calls
so functions with lots of logging are some of the worst affected.

Fixes: https://tracker.ceph.com/issues/72879
Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>