Set GZIP to -1 for tarball creation

Images will default to gzip -6, which is quite CPU intensive. -1 is faster, at a slight trade off of size. -1 appears to be 30-50% faster in real world snapshot speed, with maybe 10-20% lower compression. As this is an environment setting, I can't see it breaking anything across varying deployments. This should be a configurable option in the future, but I feel for now it's a good interim improvement to the imaging process.