Support expression mapping in iterate

[lixingwang]

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[] bug report => search github for a similar issue or PR before submitting
[*] enhancement request
[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
We cannot use expression or function in iterate
Expected behavior:
Use functions or expression in iterate field
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):