Use pkgbase

[Defenso-QTH]

Pkgbase is becoming the standard to install and upgrade FreeBSD deployments. It enables faster and more user-friendly upgrade compared to freebsd-update and would give us more granularity than extracting base.txz when creating new jails since we can choose the packages to be installed or not (e.g. we can make jails without kernel and modules).

It would be nice to see iocage adopting pkgbase too as a way to install and upgrade jails. Retiring freebsd-update-based code is probably not to be discussed yet since all users are currently using that and changing default is going to take long. We probably need to keep it as long as it is kept by FreeBSD itself.

[dgeo]

For now, I'm ok for adding a pkgbase option but I won't add this before monthes...
In any case, this should not break current jails, may make cloned jails use more storage, and I'm not sure current .pkgsave files is the last and best option (nor how to manage them or not)...