Change how packages are installed into VM images: Rather than chrooting

Change how packages are installed into VM images: Rather than chrooting
into the image and running 'pkg install' from there, use 'pkg fetch' to
download packages into a temporary location and then 'pkg add' to install
them into the image.

This simplifies the code by avoiding the need to copy /etc/resolv.conf
into the image and then delete it later, and makes it possible to cross
build (e.g., to create an amd64 image when running on i386 hardware; or
in the future for building disk images for embedded platforms).

Because pkg was implicitly installed when VM_EXTRA_PACKAGES was non-empty,
add it to VM_EXTRA_PACKAGES in azure.conf and openstack.conf to maintain
the current behaviour.

By default repo-FreeBSD.sqlite is copied into the image, (a) to match
previous behaviour, where the file would be downloaded by the chrooted
pkg invocation; and (b) because it may be useful for testing purposes,
e.g., to see why a package didn't get installed.  Because this file is
large (46 MB) and not likely to be useful in -RELEASE images which are
being launched into Clouds several months later, it can be disabled by
setting NOREPOSQLITE.

As far as I know this commit does not change the disk images produced in
any filesystem-visible way.