Increase the size of the Vagrant default image size, as the 3GB
size is too small to bootstrap the firstboot_pkgs list.

While here, add the growfs(8) startup script to /etc/rc.conf,
as Vagrant images can be resized by modifying the Vagrantfile.

Reported by: dbaio
PR: 238226
MFC after: 1 week
Sponsored by: The FreeBSD Foundation