[Tools] Add --cache-dir support to generate_gclient-xwalk.py.

The first patch is just groundwork to simplify the way we print "\n" (!)

The second one implements a way to set the "cache_dir" option in .gclient files
generated by our Python tool. It's particularly useful for our automation
setup, as it allows many machines to clone huge repositories like
blink-crosswalk in seconds.