basehub: relocate singleuser.admin -> custom.singleuserAdmin

This is needed to be compatible with z2jh 1.0.0 with schema validation
logic that will complain about unrecognized configuration options under
singleuser or anything but `custom`.

hub-templates/basehub/values.yaml

@@ -29,6 +29,11 @@ nfsPVC:
 
 jupyterhub:
   custom:
+    singleuserAdmin:
+      extraVolumeMounts:
+        - name: home
+          mountPath: /home/jovyan/shared-readwrite
+          subPath: _shared
     cloudResources:
       provider: null
       gcp:
@@ -95,11 +100,6 @@ jupyterhub:
       letsencrypt:
         contactEmail: [email protected]
   singleuser:
-    admin:
-      extraVolumeMounts:
-        - name: home
-          mountPath: /home/jovyan/shared-readwrite
-          subPath: _shared
     startTimeout: 600 # 10 mins, because sometimes we have too many new nodes coming up together
     defaultUrl: /tree
     nodeSelector:
@@ -256,7 +256,7 @@ jupyterhub:
 
         class CustomSpawner(ConfiguratorSpawnerMixin, KubeSpawner):
           def start(self, *args, **kwargs):
-            custom_admin = get_config('singleuser.admin', {})
+            custom_admin = get_config('custom.singleuserAdmin', {})
             if custom_admin and self.user.admin:
                 extra_init_containers = custom_admin.get('initContainers', [])
                 extra_volume_mounts = custom_admin.get('extraVolumeMounts', [])