Default to stable upstream versions in cros_portage_upgrade.
Default to stable upstream versions in cros_portage_upgrade.

Remove the --stable-only option, add the --unstable-ok option, and
make stable upstream versions the default behavior for cros_portage_upgrade.

BUG=chromium-os:17759
TEST=Run with --upgrade on any package with different stable/unstable upstream
versions and see which one is chosen.

Change-Id: I0c58f702ea4ab7a4f07a87b58c4f6b0cd10e48d1
Reviewed-on: http://gerrit.chromium.org/gerrit/4290
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

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