Adopt `tempesta-tech.com` in LXC container test

[mbabitski-t]

This PR resolves #507

• [x] add LXC backend for container testing
• [x] automatic LXC proxy setup and teardown
• [x] backend can optionally make a pre-test snapshot and restore the container to it's previous stage
• [x] spin up a staging site container of tempesta-tech.com
• [x] test the site with common interaction flow: get the main page, fetch list of blog posts, login as admin, create a blog post, check that the post exists, login as user, add a comment, login as admin, approve the comment and check that it appeared on the blog post page.