save metrics data to csv file

[putcn]

now aws master will save metrics data to a csv file to be able to easily integrate to other systems.
log message need to start with **metrics_data: and metrics need be in the format of comma separated KEY=VALUE form so that master will pick this line up and parse it as metric data.

example log message:
**metrics_data: Pass = 1, Training performance = 311, Training accuracy = 0.8, Test accuracy = 0.7

output csv file will look like:

Pass,  Training performance, Training accuracy, Test accuracy
1, 311, 0.8, 0.7
2, 305, 0.9, 0.8
...

[typhoonzero]

LGTM