Support construction of the graph of all reboot constraints

For online rolling reboots, there are two kind of restrictions. First,
we cannot reboot the primary and secondary nodes of an instance
together. Secondly, two nodes cannot be rebooted simultaneously, if
they are the primary nodes of two instances with the same secondary
node. The second condition requires knowledge of all nodes, not only
those the graph is to be constructed on.

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