Add --list option to list valid configs
Add --list option to list valid configs

BUG=chromium-os:16939
TEST=ran with the option.

Change-Id: I00aab5b7965d0dfbb422753c3e73d01f314a0eff
Reviewed-on: http://gerrit.chromium.org/gerrit/3153
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

coreos/chromite,zhang0137/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite