Vulcan subdomain takeover

[seilagamo]

It's necessary to implement a check to detect potential subdomain takeovers.

The flow that generates the vulnerability is:

• Person X generates a DNS record with a subdomain of ours pointing to an AWS resource.
• Person X decides to delete the resource for whatever reason without deleting the DNS record.
• An attacker identifies that there is a DNS record with a subdomain of ours pointing to an AWS resource that does not exist.
• The attacker decides to register an AWS resource with the identifier pointed to by the DNS record.

[jesusfcr]

Great job.

I added some minor comments, but the main concern is that it doesn't take into consideration multi region.

Apart from that I think the name of the check is too generic for the current implementation. One posibility could be to change to vulcan-aws-subdomain-takeover. Other posibility could be to include this functionality in the current vulcan-aws-alerts check.

Apart from that, I probably base the PR on master and not to checkshttp to prevent blockings.