Add reading username attribute from configuration file

To map correct shib attribute to username, use username attribute in
configuration file.
Allow logout to root instead of referrer URL

This commit changes the redirect URL after logout. The original logout
redirect URL was referrer URL. This causes user being redirected back to
login page, which is an unwanted behaviour. Now the user is redirect to
root "/" of the site.

Todo: using logout.target attribute in config.yml file instead of "/"