Add production setup to LXD

This are the recommendations for LXD production setup: https://github.com/lxc/lxd/blob/master/doc/production-setup.md


- Install LXD snap: the snap that already takes care of /etc/security/limits.conf
- Use ZFS storage driver: It is much faster than "dir"
- Apply sysctl recommended values

Change-Id: Id3c83a34cd853155e795ff117520911514b19107
Signed-off-by: David Garcia <david.garcia@canonical.com>