Pre-generate updates with root user.
Pre-generate updates with root user.

image_to_live runs dev_server with sudo so old cached entries are
owned by root rather than $USER.  Change to pre-generate also with
sudo.

Change-Id: I587f15953b37d4de439977e7dbac9fced175c173

BUG=chromium-os:10723
TEST=Ran it

Review URL: http://codereview.chromium.org/6380008

vmware/coreos-scripts,fivethreeo/scripts,vmware/coreos-scripts,nicescale/scripts,gtank/scripts,fivethreeo/scripts,smilart/scripts,zhang0137/scripts,mischief/scripts,bcwaldon/coreos-scripts,zhang0137/scripts,sigma/coreos-scripts,crawford/scripts,bcwaldon/coreos-scripts,mischief/scripts,mischief/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,marineam/coreos-scripts,marineam/coreos-scripts,andrejro/coreos--scripts,marineam/coreos-scripts,trnubo/scripts,BugRoger/scripts,smilart/scripts,glevand/coreos--scripts,nicescale/scripts,crawford/scripts,smilart/scripts,coreos/scripts,trnubo/scripts,mjg59/scripts,BugRoger/scripts,endocode/scripts,mjg59/scripts,BugRoger/scripts,zhang0137/scripts,glevand/coreos--scripts,endocode/scripts,gtank/scripts,endocode/scripts,andrejro/coreos--scripts,fivethreeo/scripts,gtank/scripts,coreos/scripts,andrejro/coreos--scripts,nicescale/scripts,mjg59/scripts,sigma/coreos-scripts,trnubo/scripts,crawford/scripts,sigma/coreos-scripts