Get whole attribute struct when map subflow activity's output

[lixingwang]

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

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?

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

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

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