UNSTABLE: test the dynamic tree partitionning with multiple Ceph MDS

Quick tip to enable the dynamic subtree tree partitionning with multiple Ceph MDS servers.

If you want this to take effect during cluster creation edit your ceph.conf:

[mds]
mds max =  5

Then restart your MDSs, they will be all active.

Or inject the following command:

bash $ ceph mds set_max_mds 5 max_mds = 5

Before and after:

```bash $ ceph -s

cluster 04c9ddfb-a420-4fac-bbee-997fd182f527
 health HEALTH_OK
 monmap e1: 1 mons at {ceph001=192.168.0.100:6789/0}, election epoch 1, quorum 0 ceph001
 mdsmap e20: 1/1/1 up {0=ceph006=up:active}, 4 up:standby
 osdmap e103: 29 osds: 29 up, 29 in
  pgmap v18261: 1216 pgs, 4 pools, 2145 bytes data, 23 objects
        1237 MB used, 26771 GB / 26773 GB avail
               6 active+clean+replay
            1210 active+clean

$ ceph -s

cluster 04c9ddfb-a420-4fac-bbee-997fd182f527
 health HEALTH_OK
 monmap e1: 1 mons at {ceph001=192.168.0.100:6789/0}, election epoch 1, quorum 0 ceph001
 mdsmap e30: 5/5/5 up {0=ceph006=up:active,1=ceph005=up:active,2=ceph003=up:active,3=ceph002=up:active,4=ceph004=up:active}
 osdmap e103: 29 osds: 29 up, 29 in
  pgmap v18274: 1216 pgs, 4 pools, 5893 bytes data, 85 objects
        1239 MB used, 26771 GB / 26773 GB avail
            1216 active+clean

```

Now enable the subtree partitionning with the following flag:

[mds]
mds bal frag = true

For all the options.