Add parser as an argument and parse_args hook in preparation to move parse_args at the right place, inside the CMDxx functions.

R.I.P. gclient_test.py

Review URL: http://codereview.chromium.org/2253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@48271 0039d316-1c4b-4281-b951-d872f2087c98