client: handle 0.8 server network resources

Fixes #5587

When a nomad 0.9 client is handling an alloc generated by a nomad 0.8
server, we should check the alloc.TaskResources for networking details
rather than task.Resources.

We check alloc.TaskResources for networking for other tasks in the task
group [1], so it's a bit odd that we used the task.Resources struct
here.  TaskRunner also uses `alloc.TaskResources`[2].

The task.Resources struct in 0.8 was sparsly populated, resulting to
storing of 0 in port mapping env vars:

```
vagrant@nomad-server-01:~$ nomad version
Nomad v0.8.7 (21a2d93+CHANGES)
vagrant@nomad-server-01:~$ nomad server members
Name                    Address      Port  Status  Leader  Protocol  Build  Datacenter  Region
nomad-server-01.global  10.199.0.11  4648  alive   true    2         0.8.7  dc1         global
vagrant@nomad-server-01:~$ nomad alloc status -json 5b34649b | jq '.Job.TaskGroups[0].Tasks[0].Resources.Networks'
[
  {
    "CIDR": "",
    "Device": "",
    "DynamicPorts": [
      {
        "Label": "db",
        "Value": 0
      }
    ],
    "IP": "",
    "MBits": 10,
    "ReservedPorts": null
  }
]
vagrant@nomad-server-01:~$ nomad alloc status -json 5b34649b | jq '.TaskResources'
{
  "redis": {
    "CPU": 500,
    "DiskMB": 0,
    "IOPS": 0,
    "MemoryMB": 256,
    "Networks": [
      {
        "CIDR": "",
        "Device": "eth1",
        "DynamicPorts": [
          {
            "Label": "db",
            "Value": 21722
          }
        ],
        "IP": "10.199.0.21",
        "MBits": 10,
        "ReservedPorts": null
      }
    ]
  }
}
```

Also, updated the test values to mimic how Nomad 0.8 structs are
represented, and made its result match the non compact values in
`TestEnvironment_AsList`.

[1] https://github.com/hashicorp/nomad/blob/24e9040b18a4f893e2f353288948a0f7cd9d82e4/client/taskenv/env.go#L624-L639
[2] https://github.com/hashicorp/nomad/blob/master/client/allocrunner/taskrunner/task_runner.go#L287-L303

client/taskenv/env.go

@@ -544,15 +544,9 @@ func (b *Builder) setTask(task *structs.Task) *Builder {
 	if task.Resources == nil {
 		b.memLimit = 0
 		b.cpuLimit = 0
-		b.networks = []*structs.NetworkResource{}
 	} else {
 		b.memLimit = int64(task.Resources.MemoryMB)
 		b.cpuLimit = int64(task.Resources.CPU)
-		// Copy networks to prevent sharing
-		b.networks = make([]*structs.NetworkResource, len(task.Resources.Networks))
-		for i, n := range task.Resources.Networks {
-			b.networks[i] = n.Copy()
-		}
 	}
 	return b
 }
@@ -622,6 +616,15 @@ func (b *Builder) setAlloc(alloc *structs.Allocation) *Builder {
 			}
 		}
 	} else if alloc.TaskResources != nil {
+		if tr, ok := alloc.TaskResources[b.taskName]; ok {
+			// Copy networks to prevent sharing
+			b.networks = make([]*structs.NetworkResource, len(tr.Networks))
+			for i, n := range tr.Networks {
+				b.networks[i] = n.Copy()
+			}
+
+		}
+
 		for taskName, resources := range alloc.TaskResources {
 			// Add ports from other tasks
 			if taskName == b.taskName {

client/taskenv/env_test.go

@@ -228,12 +228,11 @@ func TestEnvironment_AsList_Old(t *testing.T) {
 				Device: "eth0",
 				IP:     "192.168.0.100",
 				ReservedPorts: []structs.Port{
-					{Label: "admin", Value: 5000},
 					{Label: "ssh", Value: 22},
 					{Label: "other", Value: 1234},
 				},
 				MBits:        50,
-				DynamicPorts: []structs.Port{{Label: "http"}},
+				DynamicPorts: []structs.Port{{Label: "http", Value: 2000}},
 			},
 		},
 	}
@@ -244,10 +243,10 @@ func TestEnvironment_AsList_Old(t *testing.T) {
 			Networks: []*structs.NetworkResource{
 				{
 					Device:        "eth0",
-					IP:            "192.168.0.100",
-					ReservedPorts: []structs.Port{{Label: "admin", Value: 5000}},
+					IP:            "127.0.0.1",
+					ReservedPorts: []structs.Port{{Label: "https", Value: 8080}},
 					MBits:         50,
-					DynamicPorts:  []structs.Port{{Label: "http", Value: 2000}},
+					DynamicPorts:  []structs.Port{{Label: "http", Value: 80}},
 				},
 			},
 		},
@@ -270,10 +269,11 @@ func TestEnvironment_AsList_Old(t *testing.T) {
 		"taskEnvKey": "taskEnvVal",
 	}
 	task.Resources.Networks = []*structs.NetworkResource{
+		// Nomad 0.8 didn't fully populate the fields in task Resource Networks
 		{
-			IP:            "127.0.0.1",
-			ReservedPorts: []structs.Port{{Label: "http", Value: 80}},
-			DynamicPorts:  []structs.Port{{Label: "https", Value: 8080}},
+			IP:            "",
+			ReservedPorts: []structs.Port{{Label: "https"}},
+			DynamicPorts:  []structs.Port{{Label: "http"}},
 		},
 	}
 	env := NewBuilder(n, a, task, "global").SetDriverNetwork(