Added route table support for subnets

[tanmay-j]

This PR closes #1154

The changes in this PR are as follows:

• Add Route Table property support to Subnet Resource.

I have read the contributing guidelines and have completed the following:

• Tested my code end-to-end against a live Azure subscription.
• Updated the documentation in the docs folder for the affected changes.
• Written unit tests against the modified code that I have made.
• Updated the release notes with a new entry for this PR.
• Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

let myRt = routeTable { name "my-rt" }
let myNet = vnet {
    name vnetName
    build_address_spaces [
        addressSpace {
            space "10.28.0.0/16"

            subnets [
                subnetSpec {
                    name "webSubnet"
                    size 24
                    route_table myRt
                }
            ]
        }
    ]
}

[ninjarobot]

Looks great, thanks for including tests and doc updates.