Extend the simulation backend to also simulate a master node

In a simulated cluster as created by the simulation backend to
the htools, make the first node of the first node group the master
node. In this way, hools (like hroller) that require a master node
can also be used on simulated clusters, e.g., for testing.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>