Support Elastic IP in the ec2-controller

**Is your feature request related to a problem?**
I need an Elastic IP to allocate for a `NATGateway` in my private subnets. This is supported in the AWS SDK for Go as [`AllocateAddress`](https://docs.aws.amazon.com/sdk-for-go/api/service/ec2/#EC2.AllocateAddress).

**Describe the solution you'd like**
A new `ElasticIP` or `EIP` resource
