task/ceph: Invoke newfs for CephFS as needed

New ceph versions will no longer create the
CephFS pools and will not enable the filesystem
by default.  This change adds an explicit step
to the cluster initialization to create the
filesystem if it doesn't already exist.

Signed-off-by: John Spray <john.spray@inktank.com>
task: update 'newfs' users to 'fs new'

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #278 from ceph/wip-nullfs

Support the new fs syntax.

Reviewed-by: Greg Farnum <greg@inktank.com>