roles/common: Use a persistent systemd journal

The default systemd journal configuration on CentOS 7 and Ubuntu
16.04 does not keep journal logs for multiple boots. This limits
the usefulness of the journal entirely (for example, try to see
sshd logs from even two or three months ago!).

Changing the storage to "persistent" makes systemd keep the logs
on disk in /var/log/journal for up to 2% of the partition size.

roles/common/tasks/main.yml

@@ -31,6 +31,16 @@
   import_tasks: sshd.yml
   tags: sshd
 
+# this should work on CentOS 7+ and Ubuntu 16.04+
+- block:
+  - name: Create systemd-journald drop-in config directory
+    file: path=/etc/systemd/journald.conf.d owner=root group=root mode=0755 state=directory
+
+  - name: Enable persistent systemd journal
+    copy: src=00-persistent-journal.conf dest=/etc/systemd/journald.conf.d/00-persistent-journal.conf owner=root group=root mode=0644
+  when: ansible_service_mgr == 'systemd'
+  tags: systemd-journald
+
 # containers identify as virtualization hosts, which makes this tricky, because we have actual Debian VM hosts!
 - name: Reconfigure /etc/sysctl.conf
   when: ansible_virtualization_role != 'host'