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
Facepalm for the slipped parenthesis. I didn't catch it through `find_eip_changes`, but saw it in the log only later today.