[tune] Add options to remove default logger.

[24hours]

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Ubuntu 16.04
• Ray installed from (source or binary): pip install ray
• Ray version: 0.6.1
• Python version: 3.6.7
• Exact command to reproduce:

Describe the problem

Currently running experiment in ray tune run_experiments will automatically produce log
using [_JsonLogger, _TFLogger, _CSVLogger]. I added my own customer logger to format TFboard output into my own liking and realize that _TFLogger is still active thus cluttering my TFboard. Should we turn off default logger when custom logger is present or add new flag to turn them off.

[richardliaw]

OK, we can have a flag to opt-out of default loggers. so this would be like

run_experiments(..., default_loggers=False)

Would you be interested in posting a PR for this?