Our target is our server to enable by default clean URL at any Drupal 8 project.
If you've installed IspConfig opensource project, you probably have Apache mod_rewrite enabled. This is one of requirements. Then, you just should edit /etc/apache2/apache2.conf file and add there:
# Drupal 8: clean URL
# https://www.drupal.org/getting-started/clean-urls#comment-8780263
<Directory /var/www/clients/*/*/web/>
RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
</Directory>
Pay attention at path wildcard: /var/www/clients/*/*/web/ what let us apply this settings to further web projects at server.
After it, you should restart Apache service, to enable changes for Apache daemon:
service apache2 restart
And you get it!
20151127 Update:
In a seconfd fresh install in the same server, it seems clean url are not enables. I've solved it adding these lines to local .htaccess:
<Directory /var/www/clients/*/*/web/>
RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
</Directory>