[testing] Add support for mocks

[pgavlin]

These changes add simple support for defining mocks alongside a
template.

[pgavlin]

@AaronFriel @iwahbe @lukehoban

I wrote this up to do some local testing. I don't think that we want to take it as-is, but I wanted to capture the idea in a PR.

[pgavlin]

here's an example of what this looks like in a template:

resources:
  securityGroup:
    type: aws:ec2:SecurityGroup
    properties:
      ingress:
        - protocol: tcp
          fromPort: 0
          toPort: 0
          cidrBlocks:
          - 0.0.0.0/0
  server:
    type: aws:ec2:Instance
    properties:
      tags:
        Name: web-server-www
      instanceType: t2.micro
      securityGroups:
        - ${securityGroup.name}
      ami: ${ami}
      userData: |
        #!/bin/bash
        echo "Hello, World!" > index.html
        nohup python -m SimpleHTTPServer 80 &
variables:
  ami:
    Fn::Invoke:
      Function: aws::getAmi
      Arguments:
        filters:
          - name: name
            values:
            - amzn-ami-hvm-*-x86_64-ebs
        owners:
          - 137112412989
        mostRecent: true
      Return: id
outputs:
  publicIp: ${server.publicIp}
  publicHostName: ${server.publicDns}

mocks:
  resources:
    securityGroup:
      id: securityGroup
      state:
        name: securityGroup
    server:
      id: my-server
      state:
        publicIp: 1.2.3.4
        publicDns: my-server.com
  calls:
    "aws::getAmi":
      return:
        id: some-ami-id

[AaronFriel]

This would be very useful to put behind an env var and also as a helper method for tests. Very clean.