fix(admin): a default VM that is not exactly one VM is refused on the form

NEW_USER_DEFAULT_INSTANCES names VMs, and a name may repeat across hosts.
A name with no VM, or with one on two hosts, made creating any user a 500.
The create form now looks the names up and says which one is not exactly
one VM; the user and its default VMs are saved together.

admin/forms.py

@@ -1,4 +1,5 @@
 from django import forms
+from django.conf import settings
 from django.contrib.auth import password_validation
 from django.contrib.auth.forms import AdminPasswordChangeForm
 from django.contrib.auth.models import Group, User
@@ -7,6 +8,7 @@ from django.utils.translation import gettext_lazy as _
 
 from accounts.forms import UniqueEmailMixin
 from accounts.models import UserAttributes
+from instances.models import Instance
 
 from .models import Permission
 
@@ -85,6 +87,17 @@ class UserCreateForm(UserForm):
             content_type__app_label="accounts", codename="change_password"
         )
 
+    def clean(self):
+        # the VMs a new user owns, by name: a name may repeat across hosts
+        cleaned = super().clean()
+        self.default_instances = []
+        for name in settings.NEW_USER_DEFAULT_INSTANCES:
+            found = list(Instance.objects.filter(name=name)[:2])
+            if len(found) != 1:
+                raise ValidationError(_("NEW_USER_DEFAULT_INSTANCES: %(name)s is not exactly one VM") % {"name": name})
+            self.default_instances += found
+        return cleaned
+
     def _post_clean(self):
         # The validators in AUTH_PASSWORD_VALIDATORS, as on the password change
         # forms; after the instance has the form's values (username, email).