Import OS-specific vars from task in common role

We stopped being able to do dynamic includes from the playbooks around
Ansible 2.4.0.0 if I recall correctly. Instead we can create a task to
include the variables and make it always run by using the special tag.

For now the Debian and Ubuntu vars files are the same, but I will keep
them separate so that it is more flexible in the future.

roles/common/tasks/main.yml

@@ -1,4 +1,7 @@
 ---
+- name: Import OS-specific variables
+  include_vars: "vars/{{ ansible_distribution }}.yml"
+  tags: always
 
 - name: Configure network time
   import_tasks: ntp.yml