Add a test for online rolling reboot scheduling

In the example configuration, the graph constructed by just connecting
primary and secondary instances is two-colorable. However, when taking
conflicting locations of secondary nodes into account, three reboot
groups are needed. Moreover, these reboot groups are not subordinated
to any two-coloring of the first-mentioned graph.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>