contents-runtime: Add all timezone data

We want to support all of them.
contents: Create a default /etc/hosts

Ensure we have a default configuration for localhost.
contents-runtime: Add btrfs-tools

One day we'll use it as the default file system instead of ext4,
meanwhile we add it to the image in case users need to mount btrfs
file systems.
contents: Preserve /opt, /srv across upgrades

Make /opt and /srv symlinks to /var/opt and /var/srv, respectively.
Administrators can do whatever they want with them because they are in
/var; maui-ostree-integration creates /var/opt and /var/srv as empty
directories, thus they can be mountpoints for other partitions or
whatever suits the administrators need.