Fix problem with systemd logs not being persisted

During the boot with ostree, the detection of whether the logs on
the system should be persisted can be confused. Explicitely configure
them to be persisted.

Copied from atomic-ws.

post.sh

@@ -1,3 +1,8 @@
 #!/bin/bash
 
 set -xeuo pipefail
+
+# See https://bugzilla.redhat.com/show_bug.cgi?id=1265295 ; some
+# aspects of that have been fixed, but apparently this is still
+# necessary, and generally makes things less finicky
+echo 'Storage=persistent' >> /etc/systemd/journald.conf