Simpler lxc-gentoo for use as a lxc-create template

Hi,

this pull request is more of a "hey, look at this" rather that an  "please merge this work"

I overtook this work some months ago as I wanted to have a gentoo template that would integrate well with lxc-create just like debian has. I have hence modified quite a bit the original script in order to achieve a few things:
 * be closer to the style of other templates
 * just do what would bring a default lxc online

The rest is imho up to the sysadmin with tools like puppet/salt which is how I intend to manage such lxc guests.

This script can be directly copied into the templates directory of lxc and be invoked by lxc-create -t lxc-gentoo-simple with the other usual options.

Feel free to rip everything you find nice from it, I try to keep it up to date wrt to your repository when I have time.