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
6 files changed