Add a test demonstrating the --node-tags option of hroller

The example is a cluster of 6 nodes, paired into 3 group by three
instances. So the whole cluster would need two reboot groups. The two
tags select, in two different ways, one node of each group. So, when
restricting to one tag, a single reboot group suffices, but no
coloring of the whole cluster would achieve this.

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