Store 'networkMode' in ECS Task Definitions instead of just throwing it away

tests/test_ecs/test_ecs_boto3.py

@@ -94,6 +94,7 @@ def test_register_task_definition():
                 "logConfiguration": {"logDriver": "json-file"},
             }
         ],
+        networkMode='bridge',
         tags=[
             {"key": "createdBy", "value": "moto-unittest"},
             {"key": "foo", "value": "bar"},
@@ -124,6 +125,7 @@ def test_register_task_definition():
     response["taskDefinition"]["containerDefinitions"][0]["logConfiguration"][
         "logDriver"
     ].should.equal("json-file")
+    response['taskDefinition']['networkMode'].should.equal('bridge')
 
 
 @mock_ecs