Since using apache-2.4[1] the Include directive will fail if no glob
matches, this switch to new, IncludeOptional[2], available since 2.4, in
order to not fail if no configurations are present.

 [1] 1f7bf3d1
 [2] http://httpd.apache.org/docs/2.4/mod/core.html#includeoptional

/reviewed-on nexedi/slapos!252