Plumb in subcommand options

Enable passing of options to subcommands by avoiding any parsing of these
at the top level.

BUG=chromium-os:11785
TEST=chromite build --clean x86-generic, check it does clean
python main_unittest.py  (run tests)

Change-Id: I4bfe86bc02d12aa207a294d143201e20875fba03

Review URL: http://codereview.chromium.org/6720024
4 files changed