from_json always creates mappings with host port

[mikekatica]

When generating a MarathonApp object (or any object between it and MarathonContainerPortMapping) from a dict using from_json, a host port of 0 is always included. This causes undesired behavior when not trying to map a container port to a host port, especially in DCOS, where the overlay network can be used instead. See below:

{
  "id": "/uhttpd",
  "backoffFactor": 1.15,
  "backoffSeconds": 1,
  "container": {
    "type": "DOCKER",
    "volumes": [],
    "docker": {
      "image": "fnichol/uhttpd",
      "forcePullImage": false,
      "privileged": false,
      "parameters": [],
      "portMappings": [
        {
          "containerPort": 80,
          "servicePort": 10087,
          "name": "http"
        }
      ]
    }
  },
  "cpus": 0.1,
  "disk": 0,
  "healthChecks": [
    {
      "gracePeriodSeconds": 60,
      "ignoreHttp1xx": false,
      "intervalSeconds": 5,
      "maxConsecutiveFailures": 3,
      "portIndex": 0,
      "timeoutSeconds": 20,
      "delaySeconds": 5,
      "protocol": "HTTP",
      "path": "/"
    }
  ],
  "instances": 1,
  "labels": {
    "HAPROXY_GROUP": "internal"
  },
  "maxLaunchDelaySeconds": 3600,
  "mem": 128,
  "gpus": 0,
  "networks": [
    {
      "name": "dcos",
      "mode": "container"
    }
  ],
  "requirePorts": false,
  "upgradeStrategy": {
    "maximumOverCapacity": 1,
    "minimumHealthCapacity": 1
  },
  "killSelection": "YOUNGEST_FIRST",
  "unreachableStrategy": {
    "inactiveAfterSeconds": 300,
    "expungeAfterSeconds": 600
  },
  "fetch": [],
  "constraints": []
}

Creates the following MarathonContainer:

MarathonContainer::{"docker": {"forcePullImage": false, "image": "fnichol/uhttpd", "parameters": [], "portMappings": [{"containerPort": 80, "hostPort": 0, "labels": null, "name": "http", "protocol": "tcp", "servicePort": 10087}], "privileged": false}, "type": "DOCKER", "volumes": []}

But should create:

MarathonContainer::{"docker": {"forcePullImage": false, "image": "fnichol/uhttpd", "parameters": [], "portMappings": [{"containerPort": 80, "labels": null, "name": "http", "protocol": "tcp", "servicePort": 10087}], "privileged": false}, "type": "DOCKER", "volumes": []}