Fix vpn migrations with newer django/django-polymorphic

vpn 0001 migration was buggy, on fresh install only.

Squashing the two migrations does the trick and

That is a workaround but it covers all the cases:

- fresh install: use only the squashed
- up-to-date install: will do nothing
- installs being at 0001 : will run the 0002 (which is not the failing one)

vpn/migrations/0001_squashed_0002_remove_vpnconfiguration_comment.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import netfields.fields
+import coin.validation
+import coin.mixins
+
+
+class Migration(migrations.Migration):
+
+    replaces = [('vpn', '0001_initial'), ('vpn', '0002_remove_vpnconfiguration_comment')]
+
+    dependencies = [
+        ('configuration', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='VPNConfiguration',
+            fields=[
+                ('configuration_ptr', models.OneToOneField(parent_link=True, auto_created=True, primary_key=True, serialize=False, to='configuration.Configuration')),
+                ('activated', models.BooleanField(default=False, verbose_name='activ\xe9')),
+                ('login', models.CharField(help_text='leave empty for automatic generation', unique=True, max_length=50, verbose_name='identifiant', blank=True)),
+                ('password', models.CharField(max_length=256, null=True, verbose_name='mot de passe', blank=True)),
+                ('ipv4_endpoint', netfields.fields.InetAddressField(validators=[coin.validation.validate_v4], max_length=39, blank=True, help_text='Adresse IPv4 utilis\xe9e par d\xe9faut sur le VPN', null=True, verbose_name='IPv4')),
+                ('ipv6_endpoint', netfields.fields.InetAddressField(validators=[coin.validation.validate_v6], max_length=39, blank=True, help_text='Adresse IPv6 utilis\xe9e par d\xe9faut sur le VPN', null=True, verbose_name='IPv6')),
+            ],
+            options={
+                'verbose_name': 'VPN',
+            },
+            bases=(coin.mixins.CoinLdapSyncMixin, 'configuration.configuration'),
+        ),
+        migrations.CreateModel(
+            name='LdapVPNConfig',
+            fields=[
+            ],
+            options={
+                'managed': False,
+            },
+        ),
+    ]