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
Merge pull request #1038 from edx/jarv/vpc-dns

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