webserver: remove broken docroot logic

The current webserver::apache::site logic is broken, as it tried to
reassign to the $docroot variable and this is, unfortunately, disallowed
in puppet.

As it's clearly not a used functionality, and since Apache throws
warnings if a directory doesn't exist, remove the whole piece of code
and let vhosts that e.g. don't need a document root (because, for
example, contain just a simple redirect) to work with a simpler config.

Debian defaults to /var/www in this case, and policy dictates that no
packages will ever write useful files there.

Change-Id: I64ee6d8939b8ca56abd21bad7c76c9684cc54812