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/roles/ceph_mon/defaults/main.yml b/roles/ceph_mon/defaults/main.yml
index f001f84..86c1717 100644
--- a/roles/ceph_mon/defaults/main.yml
+++ b/roles/ceph_mon/defaults/main.yml
@@ -14,7 +14,7 @@
 # .. envvar:: ceph_mon_group [[[
 #
 # Name of Ansible group that contains all Ceph monitors
-ceph_mon_group: ceph_mons
+ceph_mon_group: controllers
 
                                                                    # ]]]
 # .. envvar:: ceph_mon_cluster_network [[[