Squashed 'elasticluster/providers/ansible-playbooks/' changes from e6e9837..3b7c416

3b7c416 Fix slurm/slurmdbd for CentOS
df17e30 Install slurmdbd on a SLURM cluster and configure basic Priority. Fixes issue #3
82274c0 Specify how many CPUs each compute node has. This is useful when using a `SelectType` different from `select/linear`, because otherwise SLURM will not be able to automatically discover the amount of CPUs (cfr. https://computing.llnl.gov/linux/slurm/faq.html#fast_schedule)

git-subtree-dir: elasticluster/providers/ansible-playbooks
git-subtree-split: 3b7c41691a6804cda3f2612e5c55bab936759e15
Merge commit '1b4f44613d52e8f28820bf75129e6fd907985b56'
Fix issue when extra variables are defined in a cluster section, not being saved in the ansible inventory file.