
On CentOS 6.x the rpm executable is in /bin instead of /usr/bin, but the cache-devstack element ends up running yum if it wants to download RPM packages anyway. Look for yum instead of rpm as an indication of which packages to install. Commit fa18656 fixed this in cache_devstack.py for snapshot images, but missed that it was affecting diskimage-builder elements as well. Change-Id: I4b76a48564cd5d703d79cfda88bbbfe97216a70a