Provide methods to create and delete VPC Subnets

A provider for provisioning subnets within an AWS VPC Environment.

```
  subnet "provisioning-vpc-subnet-a" do
    cidr_block "10.0.1.0/26"
    vpc "provisioning-vpc"
    availability_zone "eu-west-1a"
  end
```

Omitting `availablity zone` would have Amazon pick the AZ for you.