infra-journal-remote: Skip playbook on empty emtpy log_hosts group

If the log_hosts group is empty, then the deployment fails with the
following error:

{{ hostvars[groups['log_hosts'][0]]['ansible_host'] }}: list object has no element 0"}

As such, we need to skip the playbook if no hosts are defined in that
group.

Change-Id: Icf528f8c71e61de9e5a4c1e4004f5b269508d051

playbooks/infra-journal-remote.yml

@@ -26,6 +26,11 @@
       when:
         - ansible_pkg_mgr == 'zypper'
 
+    - name: Skip playbook if log_hosts group is empty
+      meta: end_play
+      when:
+        - groups['log_hosts'] | length == 0
+
     - name: Install systemd-journal-remote
       package:
         name: "{{ systemd_journal_remote_distro_package[ansible_pkg_mgr] }}"