Add mechanism to record Elastic IPs usage

This PR adds monitoring for Elastic IPs allocated to an AWS account.

The rationale for this is to help filtering issues and review public-facing ones first. The big picture would be to link EIP to running instances and their SG, in order to easily check for externally reachable services. This would be particularly useful in case of big VPC deployment.

Automatic matching of EIP<->instance<->SG is not implemented here.

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>

cc @gvarisco
I wonder if there's a way to move this rec2.get_all_tags() outside the for-loop.  If there was a way to issue one call and filter on an entire list of resource-id's, this could run much faster.