Add Compress Type Option to VNFS

[bensallen]

Add a compression type attribute to Warewulf::VNFS and --compress CLI option to Warewulf::Module::Cli::Vnfs and wwvnfs. Dynamically select compression command based on type, i.e. if type gzip then use gzip command option.

Interest in this is due to on slower CPUs decompression time + transfer time is longer than transfer time of an uncompressed CPIO.

Testing on an Intel KNL CPU with various other compression options (lz4, lzop, xz) show gzip -9 as the best option for decompression time. I suspect this is due to the tool implementation (eg. pigz) versus a superior algorithm. As such, so far I don't see a reason to add additional algorithm support in the initramfs, but this work will open up that possibility if there ends up being a need.

This option will be used to control behavior in wwgetvnfs in the initramfs.

Behaviors/Changes:

• Default to current gzip type
• Add type none to store an uncompressed CPIO and skip the gunzip in wwgetvnfs
• Limit possible values to none, gzip, UNDEF
• Add pigz (BSD license) to initramfs to speed up decompression time

Questions:

• Set type on Warewulf upgrade (i.e. wwsh object canonicalize -t vnfs), should we set this to gzip?