luci-app-lxc: code cleanup

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>