roles/common: Use static imports for tasks

Something seems to have happened as of Ansible 2.4.0.0 where this no
longer works. I suspect it is related to the major changes to static
and dynamic imports that landed around this same time.

In practice this achieves the same function, but without the "magic"
ability to use one task for different operating systems.

roles/common/tasks/main.yml

@@ -10,11 +10,23 @@
   tags: ntp
 
 - name: Install common packages
-  include_tasks: packages_{{ ansible_distribution }}.yml
+  import_tasks: packages_Debian.yml
+  when: ansible_distribution == 'Debian'
+  tags: packages
+
+- name: Install common packages
+  import_tasks: packages_Ubuntu.yml
+  when: ansible_distribution == 'Ubuntu'
   tags: packages
 
 - name: Configure firewall
-  include_tasks: firewall_{{ ansible_distribution }}.yml
+  import_tasks: firewall_Debian.yml
+  when: ansible_distribution == 'Debian'
+  tags: firewall
+
+- name: Configure firewall
+  import_tasks: firewall_Ubuntu.yml
+  when: ansible_distribution == 'Ubuntu'
   tags: firewall
 
 - name: Configure secure shell daemon