allow disabling default vhosts under 2.4

when disabling the default vhost(s) under 2.4, there's nothing to
include in $sites-enabled/, which causes the following error:

    The apache2 configtest failed.
    Output of config test was:
    apache2: Syntax error on line 46 of /etc/apache2/apache2.conf: No
    matches for the wildcard '*' in '/etc/apache2/sites-enabled', failing
    (use IncludeOptional if required)
    Action 'configtest' failed.
    The Apache error log may have more information.

by using IncludeOptional (as recommended), we allow the use of httpd as
simple web server, with single purpose configuration, without having to
define a (default) vhost.