Upgrade to Ansible 2.9.x.

Breaking changes:

* Switched to Ansible 2.9.x, removing support for older versions. All

  documentation has been updated.

Bug fixes:

* ``common`` role

  * Run apticron at least once during initial installation to avoid

    accidental locking later on during the same playbook run.

New features/improvements:

* Tests have been updated to work with latest Molecule/Testinfra as

  part of the Ansible upgrade process.