Validate accumulate for iterator/dowhile only

[lixingwang]

Current behavior (how does the issue manifest):
Validate accumulate for iterator/do while only. we shouldn't allow the user to enable accumulate for a common tasks.
Expected behavior:

Minimal steps to reproduce the problem (not required if feature enhancement):

Please tell us about your environment (Operating system, docker version, browser & web ui version, 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):