tests: reduce max_mds from 3 to 2

tests: reduce max_mds from 3 to 2

Having max_mds value equals to the number of mds nodes generates a
warning in the ceph cluster status:

cluster:
id:     6d3e49a4-ab4d-4e03-a7d6-58913b8ec00a'
health: HEALTH_WARN'
        insufficient standby MDS daemons available'
(...)
services:
  mds:     cephfs:3 {0=mds1=up:active,1=mds0=up:active,2=mds2=up:active}'

Let's use 2 active and 1 standby mds.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>