Log Level enhancements

[ayh20]

I'm submitting a ... (check one with "x")

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

Current behavior (how does the issue manifest):
Info/Error/Debug messages have various levels, but none allow for ALL messages to be suppressed.

Expected behavior:
I'd like to see 2 parts to this. Firstly, a simple suppression of messages via the setting of the env var FLOGO_LOG_LEVEL to NONE. But secondly it would be best to have a way to set the log level dynamically so that, for example, DEBUG level could be turned on and then off without having to restart the Flogo app.

Minimal steps to reproduce the problem (not required if feature enhancement):
Run any flogo executable...

What is the motivation / use case for changing the behavior?
To suppress all messages generated by the flogo app...

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): 0.5.4

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