openqa/worker: tap workers have default classes too

we don't want these workers to *only* run tap tests, so put the
default classes into their WORKER_CLASS too.

roles/openqa/worker/templates/workers.ini.j2

@@ -1,5 +1,5 @@
 [global]
 HOST = http://{{ openqa_hostname|default('localhost') }}
 {% if openqa_tap is defined and openqa_tap %}
-WORKER_CLASS = tap
+WORKER_CLASS = tap,qemu_x86_64,qemu_i686,qemu_i586
 {% endif %}