luci-app-lxc: various changes

1) Modify dependencies

Add dependencies required for to be able use containers created from
"images.linuxcontainers.org". Several of them require "lxc-attach" for set
password so to be able login. None of them has SSH preinstalled so
"lxc-console" is required to be able login and install SSH, for example.

Remove dependency "xz", it seems incompatible with LXC_BUSYBOX_OPTIONS when
both are enabled happens a build crash.

2) Change container image repository

The repository "virtualwrt.org/containers/" seems to not work, I have change
it by the official LXC container image repository.

3) Translate the arch

Translate the local uname architecture to a valid "images.linuxcontainers.org"
arch. Only tested with the platform "mvebu" (armv7l -> armhf).

4) Other minor fixes

Use same server to list images and download the image.
Disable GPG check when listing images.

Reported-by: "Admin Localnet <localnet@users.noreply.github.com&gt;">

[Squashed commits, cleaned up whitespace, refactor arch mapping, escape url
 setting, use system wide ubus helper, use uci model library]

Closes: #1422
Signed-off-by: Jo-Philipp Wich <jo@mein.io>