upgrade_chroot: integrate upgrade hooks into chroot updater
upgrade_chroot: integrate upgrade hooks into chroot updater

BUG=chromium-os:6151
TEST=run update_chroot

Change-Id: Ieb9afafa7961bd43f9b53ead14e698adc9d5179e
Reviewed-on: http://gerrit.chromium.org/gerrit/886
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

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