move credential to prompt-for mechanism

awx/main/migrations/_ask_for_variables.py

@@ -0,0 +1,9 @@
+def migrate_credential(apps, schema_editor):
+    '''If credential is not currently present, set ask_for_credential_on_launch
+    equal to True, and otherwise leave it as the default False value.
+    '''
+    JobTemplate = apps.get_model('main', 'JobTemplate')
+    for jt in JobTemplate.objects.iterator():
+        if jt.credential is None:
+            jt.ask_credential_on_launch = True
+            jt.save()