Versions of dependencies should be managed in playbooks where possible

[manics]

At the moment most of our playbooks use the default version (as defined in the role) of dependencies such as Ice and Postgres, which means bumping any of these dependencies means checking all known playbooks and deciding whether to upgrade or limit to the previous version.

I think it would be better to decouple the playbooks, so the roles could always default to the latest supported version (e.g. Ice 3.6, postgres 9.5), and we make it clear that it's the responsibility of the playbook to define the required version if it's not the latest. This makes it easier to upgrade systems independently.

See also ome/omero-install#121