Salt: force service provider to systemd if systemd

The version of Salt we're running doesn't do a good job of detecting
systemd.  Inspired by https://github.com/saltstack/salt/issues/13926,
I added a provider-force to the services.

With this change, salt-call -l debug state.highstate succeeds, even for
repeated invocations.

The issue was (probably) benign, but definitely caused noised (e.g. #11297)

init.sls

@@ -235,3 +235,7 @@ kube-addons:
 {% else %}
       - file: /etc/init.d/kube-addons
 {% endif %}
+{% if pillar.get('is_systemd') %}
+    - provider:
+      - service: systemd
+{%- endif %}