library/roles/fail2ban: Fix the tests on the distributions versions, again.

infrastructure-services/group_vars/images_provider/main.yml: Install the linux kernel headers, otherwise the virtualbox modules aren't compiled.

fail2ban/tasks/main.yml

@@ -1,5 +1,3 @@
 ---
 - include: fail2ban.yml
-  when:
-    - is_trusty
-    - is_debian8
+  when: ( is_trusty ) or ( is_debian8 )

ubuntu-deb-general/tasks/main.yml

@@ -10,7 +10,9 @@
 - include: disable-ipv6-old-servers.yml
   when: disable_ipv6
 - include: denyhost.yml
-  when: is_debian_7_or_older or is_ubuntu_less_than_trusty
+  when:
+    - is_debian_7_or_older
+    - is_ubuntu_less_than_trusty
 - include: munin.yml
   when: configure_munin
 - include: pubkeys.yml