luci-app-lxc: fix "plain-vanilla" integration

I've tried to get the lxc app in a more usable state. Tested with mips
and amd64 targets.

* fix missing tar/xz dependency,
  only if LXC_BUSYBOX_OPTIONS is not selected
* mute needless gpg validation warning
* tidy up controller a little bit
* fix multiple possible dispatcher errors
* fix compatibility with XHTML standard theme (looks still horrible ;-)
* inform the user about custom kernel prerequisites
* inform the user about the template download
* inform the user if no template was found

Signed-off-by: Dirk Brenken <dev@brenken.org>