Get whole attribute struct when map subflow activity's output

[lixingwang]

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Current behavior (how does the issue manifest): Map subflow activity's output to a log message and it get 2019-01-03 15:27:42.856 INFO [activity-flogo-log] - {"name":"output2","type":"string","value":"subflow output"} it should be only value, eg subflow output Expected behavior: Just get subflow output's value rather than whole attribute struct. Minimal steps to reproduce the problem (not required if feature enhancement):

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

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Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

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