Updating DNS script for multiple accounts in the edx.org zone

This PR makes the following changes:

* Defaults to the edx.org zone instead of a vpc zone
* Takes an AWS id and secret to query another account for instance info
* Adds some safe guards around duplicate dns entries
* Creates an ban list for elbs

util/vpc-tools/vpcutil.py

 import boto
 
-def vpc_for_stack_name(stack_name):
-    cfn = boto.connect_cloudformation()
+def vpc_for_stack_name(stack_name, aws_id=None, aws_secret=None):
+    cfn = boto.connect_cloudformation(aws_id, aws_secret)
     resources = cfn.list_stack_resources(stack_name)
     for resource in resources:
       if resource.resource_type == 'AWS::EC2::VPC':
@@ -9,13 +9,13 @@ def vpc_for_stack_name(stack_name):
 
 def stack_name_for_vpc(vpc_name):
     cfn_tag_key = 'aws:cloudformation:stack-name'
-    vpc = boto.connect_vpc()
+    vpc = boto.connect_vpc(aws_id, aws_secret)
     resource = vpc.get_all_vpcs(vpc_ids=[vpc_name])[0]
     if cfn_tag_key in resource.tags:
         return resource.tags[cfn_tag_key]
     else:
         msg = "VPC({}) is not part of a cloudformation stack.".format(vpc_name)
         raise Exception(msg)
-    
-    
+
+