prereq: no need to explicitly install python-apt

Ansible 1.4 and higher will automatically install this if needed.
Bail early if Ansible version not high enough

We need Ansible 1.4.1 (or will do shortly) in order to ensure that we're
not hit by bug #5010:

  https://github.com/ansible/ansible/issues/5010

This is relevant because of some pending changes I intend to merge to
users management.
Normalise formatting of main.yml

And while we're at it, don't overspecify the configuration by including
options that are Ansible defaults, like

  connection: ssh

or

  gather_facts: true
Replace 'base-role' with inlined all-host roles

There's no need for the indirection added by base-role when we can just
ensure that certain roles are run on all hosts using an "all" block.