let us override the tar command

Some unix (such as Mac OS X) might not provide a GNU tar command. The
new --tar-command let us override it.

On Mac one would:

 brew install gnu-tar
 ./make-release.py --tar-command gtar

And then manage to produce a tarball.

Change-Id: I556e34382c0065678341aa86ddf278bf1f536cc1