prebuilt.py: strip 'chroot-' from tarball names
prebuilt.py: strip 'chroot-' from tarball names

BUG=chromium-os:15463
TEST=unit tests

Change-Id: I36d0ffdb77cfae5d8ce7c65bc2e1f8c9c88d26d2
Reviewed-on: http://gerrit.chromium.org/gerrit/3161
Reviewed-by: Anush Elangovan <be3ea4b32dfea400cfb02e20cf45a400ef235334@chromium.org>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

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