Systematic logging support

[chungers]

Right now logging is pretty ad-hoc. Other than using logrus here and there, there is no pattern or systematic support for provisioner developers. docker-machine exposes logger for driver developers to use and libmachete should as well.

[wfarner]

Clarifying - the goal is a modest one (rather than a logging API facade); to add a GetLogger() function somewhere that makes it obvious which logger to use.

[linsun]

We really need to enhance the logging support. First of all, all logging should have timestamp. 2nd, the current code has little or no log, it is really hard to debug things when stuff goes wrong.

[wfarner]

@linsun completely agreed that logging is too sparse right now. This issue relates to a logging framework for use in plugins, so i've filed #269 to track improvements to the thoroughness of logging.

[chungers]

#440 implements this