cros_sdk: request sudo credentials before doing anything else
cros_sdk: request sudo credentials before doing anything else

BUG=chromium-os:15463
TEST=run and see

Change-Id: Iae13b56d2cdab92239ff93553b3c621ca44f82b3
Reviewed-on: http://gerrit.chromium.org/gerrit/3742
Reviewed-by: Anush Elangovan <be3ea4b32dfea400cfb02e20cf45a400ef235334@chromium.org>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

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