README update, default vars in roles, new vars

* default vars in mediatum and db roles, can be overridden in all
* all moved to all.example
* db_group var to set group for db hosts
* ansible updates: use jinja2-style vars everywhere
added pydot, config fixes, added vagrant conf

* db switch moved to role from site.yml
* added pydot and graphviz
* mediatum_admin_user var was missing
* added vagrant conf which uses the playbook for provisioning