run_chroot_version_hooks: sanity check for multiple upgrades hooks
run_chroot_version_hooks: sanity check for multiple upgrades hooks

If people land upgrade hooks of the same version levels independently and
no one notices, we get into an inconsistent state where only one of the
hooks actually runs.  Add a sanity check so we bail rather than upgrading
the chroot with an arbitrarily picked script.

BUG=None
TEST=run build_packages with multiple 17_xxx files in a ver 16 chroot and see script error out
TEST=run build_packages with single 17_xxx file in a ver 16 chroot and see successful run

Change-Id: If9a71f60badb7c9643c1a03ad49bb23ffb64f341
Reviewed-on: https://gerrit.chromium.org/gerrit/11972
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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