fix(pillars): ensure `addusers` & `delusers` are lists

pillar.example

@@ -17,8 +17,10 @@ groups:
   niceguys:
     gid: 4242
     system: false
-    addusers: root
-    delusers: toor
+    addusers:
+      - root
+    delusers:
+      - toor
   ssl-cert:
     system: true
     members:

test/salt/pillar/default.sls

@@ -17,8 +17,10 @@ groups:
   niceguys:
     gid: 4242
     system: false
-    addusers: root
-    delusers: toor
+    addusers:
+      - root
+    delusers:
+      - toor
   ssl-cert:
     system: true
     members: