purge: ensure no ceph kernel thread is present

purge: ensure no ceph kernel thread is present

This tries to first unmount any cephfs/nfs-ganesha mount point on client
nodes, then unmap any mapped rbd devices and finally it tries to remove
ceph kernel modules.
If it fails it means some resources are still busy and should be cleaned
manually before continuing to purge the cluster.
This is done early in the playbook so the cluster stays untouched until
everything is ready for that operation, otherwise if you try to redeploy
a cluster it could end up by getting confused by leftover from previous
deployment.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1337915
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>