Enable installation of snaps in to images that are built

[OddBloke]

No description provided.

[vorlonofportland]

$ snap download --help
Usage:
snap [OPTIONS] download [download-OPTIONS]

The download command downloads the given snap and its supporting assertions
to the current directory under .snap and .assert file extensions, respectively.

[...]
$

We can use this command to download snaps as part of the image build, this should not be dependent on a running snapd. I believe we would still need livecd-rootfs to move the downloaded files into the correct directories in the image where snapd would pick them up after boot.

[OddBloke]

I've confirmed that snap download works fine in a customisation script.

[OddBloke]

(As expected, snap install doesn't work:

+ snap install hello-world
2017/02/03 16:57:19.922635 main.go:220: WARNING: cannot create syslog logger
error: cannot communicate with server: Post http://localhost/v2/snaps/hello-world: dial unix /run/snapd-snap.socket: connect: no such file or directory

)