Implement LocalHeadCommandRunner.rsync_(up|down)()

[johny-b]

rsync_up/rsync_down sends files from the local machine to remote head.

In our case this is the same machine, so this doesn't-really-make-that-much sense, but we must nevertheless implement it.
Current idea:

• Have some special directory. It's TBD where this directory should be created. Maybe we can have a subdir somewhere in /tmp/ray_*, where ray holds all the stuff? Or use appdirs?
• rsync_up copies files to this directory, rsync_down copies from this directory.

So, we have a local directory we pretend is "remote".
BUT: we should start with learning when is rsync_up and rsync_down used internally by ray. E.g. if ray transfers some files this way, we must ensure it knows later how to use them.

[mateuszsrebrny]

cleaning issues (issue no longer needed)