Engine does not start when assigning an iterator to an activity

[mellistibco]

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[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
When starting the engine with an iterator attached to an activity the following error is thrown:

2018-03-28 15:18:09.971 ERROR  [main-engine] - Failed to start engine due to error: error unmarshalling flow: Unsupported task type: iterator

Expected behavior:
The engine starts and the iterator works as expected

Minimal steps to reproduce the problem (not required if feature enhancement):
Add an iterator to an object from webui, export and build the app.

          {
            "id": "counter_2",
            "type": "iterator",
            "name": "Increment Counter",
            "description": "Simple Global Counter Activity",
            "settings": {
              "iterate": "5"
            },
            "activity": {
              "ref": "github.com/TIBCOSoftware/flogo-contrib/activity/counter",
              "input": {
                "counterName": "",
                "increment": false,
                "reset": false
              },
              "mappings": {
                "input": [
                  {
                    "type": "literal",
                    "value": true,
                    "mapTo": "increment"
                  },
                  {
                    "type": "literal",
                    "value": false,
                    "mapTo": "reset"
                  },
                  {
                    "type": "assign",
                    "value": "increment",
                    "mapTo": "counterName"
                  }
                ]
              }
            }
          }

What is the motivation / use case for changing the behavior?
Resolve the bug

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):
N/A

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.5.2

Additional information you deem important (e.g. issue happens only occasionally):

[mellistibco]

Resolved in 0.5.3