Expose ports in env as PORT_XX.

Make it possible to refer to declared ports by number when
starting/configuring an app.

That means that if you put the following in your app manifest:

{
  [...]
  "ports": [80, 443]
  [...]
}

Then the env for your app will contain the following variables:

PORT_80=32185
PORT_443=32184

Port mappings are also supported, so if you put the following:

{
  [...]
  "container": {
    "type": "DOCKER",
    "docker": {
      [...]
      "network": "BRIDGE",
      "portMappings": [
        {
          "containerPort": 8080,
          "hostPort": 0,
          "servicePort": 9000,
          "protocol": "tcp"
        },
        { "containerPort": 161, "hostPort": 0, "protocol": "tcp"}
      ]
    }
  }
  [...]
}

Then these vars will be added to the env:

PORT_8080=32185
PORT_161=32184

Fixes #588