Add unit tests for hroller

Add unit tests for hroller

Add a unit test for hroller in a situation where the order
is completely determined. There are 3 nodes where node 2 has,
for each of the other nodes, an instanced shared with it. So
node 2 has to be a reboot group of its own. There are no more
instances, hence, my minimality, the other two nodes have to
form a single group. Node 1 is the master, and hence has to
be rebooted last; this determines the order.

Also add a unit test verifying that hroller rejects configurations
with multiple master nodes, even if the option --force is given.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>