Use PHP generator to prevent processing of all available site

[sfroemkenjw]

What this pr does

Use PHP generator with yield to process just the available sites needed

How to test

With over 300 root pages it needs over 30 seconds to build up all available sites. With help of a PHP generator we can stop processing all available sites if just the first site is requested. It's also helpful to just check, if there are available sites.

This should speed up performance a lot.