Simplify Ansible inventory

In order to make things more straightforward, we switch only to 3
groups:

- controllers
- cephs
- computes

We can introduce more flexiblity in the future, but this is good
enough for now.

Sem-Ver: feature
Change-Id: I450ce3f79db3eec3f5e062d3cbd4a8764081b268
diff --git a/playbooks/ceph.yml b/playbooks/ceph.yml
index 8bc3a33..58ba618 100644
--- a/playbooks/ceph.yml
+++ b/playbooks/ceph.yml
@@ -13,25 +13,20 @@
 # under the License.
 
 - name: Setup Ceph repository
-  hosts: cephs
+  hosts: controllers:cephs
   become: true
   roles:
     - ceph_repository
 
-- name: Deploy Ceph monitors
-  hosts: ceph_mons
+- name: Deploy Ceph monitors & managers
+  hosts: controllers
   become: true
   roles:
     - ceph_mon
-
-- name: Deploy Ceph managers
-  hosts: ceph_mgrs
-  become: true
-  roles:
     - ceph_mgr
 
 - name: Deploy Ceph OSDs
-  hosts: ceph_osds
+  hosts: cephs
   become: true
   roles:
     - ceph_osd