Migrated the inputs schema on existing CredentialTypes

to convert the custom become_method into a plain string.

related #2630

Signed-off-by: Jeff Bradberry <jeff.bradberry@gmail.com>

awx/main/migrations/0057_v350_remove_become_method_type.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2019-01-29 19:56
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+# AWX
+from awx.main.migrations import _credentialtypes as credentialtypes
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0056_v350_custom_venv_history'),
+    ]
+
+    operations = [
+        migrations.RunPython(credentialtypes.remove_become_methods),
+    ]