
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