update heat image to the 64bit one we cached previously

start caching the next 64bit update image to deal with heat
breaks we see from time to time.

Change-Id: If62840f4807151ada1598046fb7ad47c7210df12
Merge "update heat image to the 64bit one we cached previously"
Merge branch 'master' of https://github.com/openstack-dev/devstack