[PATCH 5/5] Integrate the layouts framework into the controller startup stage

With this commit, the slurm controller will now initialize the configured
layouts and parse their configuration files at startup.

Note that even if no layouts were configured, a "base" layout is
automatically built in slurm-layouts_load_config(). This "base" layout is
containing every nodes defined in the system, linked together as a flat tree.

(Adapted for slurm-14.11 by Matthieu Hautreux <matthieu.hautreux@cea.fr>)