Add another layer to the cache to keep regions separated

[sas-pemcne]

This fix corrects an issue where the nuker does not correctly handle resources from multiple regions in a single run.

Currently, the nuker logic, specifically in the HandleWait function, is as follows:

1. Try and get the item type out of the cache
2. If the item type does not exist, list the resources and add to the cache
3. Loop through cached resources and try to find a match for our item id
4. If our item isn't in the cache, it has been deleted

The flaw in this design is that the item.List(), called in step 2 above, only lists items for a single region (assuming the resource is region specific). Because of this, the cache will not be accurate for any region outside of the region that the item was in.

Consider the following scenario, nuker runs on two regions: us-east-1 and us-east-2. us-east-1 has a single VPC and us-east-2 has a VPC with dependencies that cannot be deleted.

1. In us-east-1, EC2VPC is processed. Given the logic above a cache is built
2. Cache ends up being an empty array because the VPC was deleted successfully on first pass
3. us-east-2 is processed next. Given the logic above, EC2VPC exists in the cache, the empty array is pulled, and no cache updates were performed
4. The VPC does not exist in the cache, thus it assumes it was deleted
5. Nuker completes successfully thinking all items were complete

Because of this, I was getting errors very similar to the following where the debug clearly shows a 400 response from AWS but the nuker thinks the resource has been removed because of the incorrect cache.

Action=DeleteVpc&Version=2016-11-15&VpcId=vpc-123

time="2019-12-20T11:18:56Z" level=debug msg="received AWS response:
< HTTP/1.1 400 Bad Request ... <Error><Code>DependencyViolation</Code><Message>The vpc 'vpc-123' has dependencies and cannot be deleted.</Message></Error> ..."

us-east-1 - EC2VPC - vpc-123 - [ID: "vpc-123", IsDefault: "false"] - removed

My fix was to simply add another layer onto the cache and have all cached items reside in their respective region. The following is the result of the cache structure after my fix:

{
  'us-east-1': {
    'EC2VPC': [],
    'EC2Instance': [],
    ...
  },
  'us-east-2': {
    'EC2Instance': [],
    ...
  },
  ...
}