Make hroller insist on finding precisely one master node

As people rely on the master node being the last node of the last
group, make hroller fail, if no master node could be found in the
cluster. This happens, e.g., if a backend format is used that does not
contain information about the master node. The error can be turned
into a warning by using the --force option.

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