Allow optional additional roles to be executed

[XaverStiensmeier]

This can be done by including roles based on an item list and by adding a new roles folder in the ansible.cfg see https://stackoverflow.com/questions/34901858/how-to-keep-local-roles-separated-from-the-ones-loaded-from-ansible-galaxy

[XaverStiensmeier]

One solution could be to add a new roles folder in ansible.cfg and then allow new roles to be added via the cluster-configuration. A first roll could be #440

[XaverStiensmeier]

this is now present in dev