Change design of algorithm for computing rolling reboots

Instead of computing a coloring for one condition first and then refining
it for the other condition, we can construct a graph with edges for all
conditions that prevent simultaneous reboots. This will not only result
in simpler code, but might also lead to better colorings.

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