[PATCH 4/5] Introduce layouts core primitives

This commit introduces the entity, layout and layouts manager related structures
corresponding to the core of the layouts logic.

The layouts manager is responsible for initializing and loading the layouts
specified in the slurm.conf "Layouts=" pragma retrieved using
slurm_get_layouts().

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