add support for logging in json format

[fm-tibco]

What kind of change does this PR introduce? (check one with "x")

[] Bugfix
[x] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

Fixes: github.com/TIBCOSoftware/flogo/issues/367

What is the current behavior?

What is the new behavior?

If you set the env variable FLOGO_LOG_FORMAT to "JSON", the json formatter will be used for logging