QA: Add config entries for virtual cluster

If these new options are set, a virtual cluster is created and used.
Virtual clusters consist of 1..N virtual nodes on a single physical (or
virtual) node. The design of virtual clusters is described in
“doc/design-virtual-clusters.rst”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>