qa/workunits/cephtool/test.sh: create pool with power-of-2 pg num

qa/workunits/cephtool/test.sh: create pool with power-of-2 pg num

otherwise wait_for_health() fails like:

wait_for_health:  ceph health detail
HEALTH_WARN 1 pool(s) have non-power-of-two pg_num
[WRN] POOL_PG_NUM_NOT_POWER_OF_TWO: 1 pool(s) have non-power-of-two pg_num
    pool 'rbd' pg_num 10 is not a power of two
../qa/workunits/cephtool/../../standalone/ceph-helpers.sh:1613: wait_for_health:  return 1

the failure was found when testing test_mon_pg().

this behavior was introduced by 6e46b1c0e50ad377d5db058e0452b0d956b3fad6

Signed-off-by: Kefu Chai <kchai@redhat.com>