Allow steps to have a load-balancer in front of them

[bbangert]

To reach parity with loads-broker, a step should be able to request a load-balancer in front of it, along with the path/port to use for the ELB health-check.

This is somewhat more involved than loads-broker, but will allow much greater control over how traffic is sent to the instances through the ELB rather than relying on DNS round-robin which loads-broker relied on by manipulating the DNS entries for a name.

Instead of letting someone create their own dns name as loads-broker did, the thought here is that when requesting an ELB, you provide an environment variable alias it will get.

For example, consider the following step in a test plan:

{
    "name": "TestCluster",
    "instance_count": 1,
    "instance_type": "t2.medium",
    "run_max_time": 140,
    "cpu_units": 2048,
    "container_name": "bbangert/pushgo:1.5rc4",
    "port_mapping": "8080:8090,8081:8081,3000:3000,8082:8082",
    "load_balancer": {
        "env_var": "TEST_CLUSTER",
        "ping_path": "/status/health",
        "ping_port": 8080,
        "ping_protocol": "http",
        "listeners": [
            {
                "listen_protocol": "ssl",
                "listen_port": 443,
                "backend_protocol": "tcp",
                "backend_port": 8080
            },
            {
                "listen_protocol": "https",
                "listen_port": 9000,
                "backend_protocol": "http",
                "backend_port": 8090
            }
        ]
    }
}

Then when a later step wants to talk to it, it can use $TEST_CLUSTER to refer to the ELB's DNS name that was assigned.

ardere can create the ELB's it finds in the steps first, then during service creation can associate the load-balancer. Once the ELB is made, we should have its DNS name back to provide to the other steps via environment vars.