Do not require an admin password on upgrade.

This commit makes us only require an admin password if there is a new
admin user.

awx/main/management/commands/user_exists.py

@@ -0,0 +1,22 @@
+# Copyright (c) 2014 Ansible, Inc.
+# All Rights Reserved
+
+import sys
+
+from django.contrib.auth.models import User
+from django.core.management.base import BaseCommand, CommandError
+
+
+class Command(BaseCommand):
+    """A command that reports whether a username exists within the
+    system or not.
+    """
+    def handle(self, *args, **options):
+        any_not_found = False
+
+        for username in args:
+            if User.objects.filter(username=username).count():
+                print('User %s exists.' % username)
+            else:
+                print('User %s does not exist.' % username)
+                any_not_found = True