Add htpasswd authentication backend

I haven't tested it properly, so this is just a quick hack to see how much problem would it be to add htpasswd file backed up authentication and if it would be possible to merge it upstream.

Reasoning: if I understand correctly, currently you have to put on every LXC node separate instance of the LWP. Editing and distributing htpasswd file for multiple machines is a little bit easier (if you are using something like puppet) then using sqlite db files.