Fix topology/tree logic when nodes defined in slurm.conf get re-ordered

From Chris Holmes, HP:
After several days of brainstorming and debugging, I have identified
a bug in SLURM 2.5.0rc2, related to the 'tree' topology. It was so
early in the execution of the whole SLURM machinery that it took me
some time to figure it out (say, 100 or 200 jobs showing the issue,
with more or less debugging levels increased and extra
instrumentation, with sometimes an uncertain reliability)...

For every “switch” a bitmap of nodes (seen down by the switch) is
built as the topology is discovered through 'topology.conf'.

There is code in read_config.c, executed when the SLURM control
daemon starts, that reorders the nodes (according to their hostname
by default), while the switches table (ie the bitmaps) has already
being built. To reorder the nodes means that the bitmaps of the switches become wrong.