This commit does multiple things:

* remove support for AllNodes where we would deploy one rgw per node on
all the nodes.
* a transition path is implemented in the code so that if someone has an
existing deployment, daemonsets will be removed and replaced by an
deployments.
* when using "instances", each rgw deployed has its own key which makes
Ceph reporting the exact number of rgw running, see:

```
[root@rook-ceph-operator-775cf575c5-bh4sr /]# ceph -s
  cluster:
    id:     611fcf39-0669-4864-9a12-debb35c0397a
    health: HEALTH_OK

  services:
    mon: 3 daemons, quorum a,b,c (age 12h)
    mgr: a(active, since 12h)
    osd: 3 osds: 3 up (since 12h), 3 in (since 12h)
    rgw: 3 daemons active (my.store.a, my.store.b, my.store.c)

  data:
    pools:   6 pools, 600 pgs
    objects: 235 objects, 3.8 KiB
    usage:   3.0 GiB used, 84 GiB / 87 GiB avail
    pgs:     600 active+clean
```

Closes: https://github.com/rook/rook/issues/2474, https://github.com/rook/rook/issues/2957 and https://github.com/rook/rook/issues/3245

Signed-off-by: Sébastien Han <seb@redhat.com>