systemd: accept .slice extension in unit-file routing

Slices are a standard systemd unit type; the existing routing only
covered timer/service/mount/swap/target and raised on .slice. Same
install path (/usr/local/lib/systemd/system/<name>) and same
systemd-reload trigger as the other unit kinds.

bundles/systemd/items.py

@@ -33,7 +33,7 @@ for name, unit in node.metadata.get('systemd/units').items():
                 'svc_systemd:systemd-networkd.service:restart',
             ],
         }
-    elif extension in ['timer', 'service', 'mount', 'swap', 'target']:
+    elif extension in ['timer', 'service', 'mount', 'swap', 'target', 'slice']:
         path = f'/usr/local/lib/systemd/system/{name}'
         dependencies = {
             'triggers': [