There was harcoded list of available images. Instead of it, LuCI LXC app will
now try to download list of available container images.

Signed-off-by: Michal Hrusecky <michal.hrusecky@nic.cz>

I makes quite some sense to provide user the choice between various versions of
the distribution to install into LXC.

Signed-off-by: Michal Hrusecky <michal.hrusecky@nic.cz>

Number of suggested rewrites:
* use lua ubus bindings instead of os.execute
* combine multiple actions to use same handler 'lxc_action' and pass
actions as arguments to consolidate code
* read openwrt version from lua directly
* start using String.prototype.format for string formatting

Also, drop 'rename' and 'clone' until they are fully functional on all
platforms.

Thanks Jo-Philipp for suggestions on how to improve the code.

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>