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.
add vpc support
set connection state ~atomically, preventing race condition in setup threads
add node core and memory amounts in slurm.conf, so we can use select/cons_res
gracefully handle vpcs, subnets with no Name tag
Allow IAM instance profiles to be passed through input configuration, enables association of IAM roles with clusters to avoid passing credentials
add vpc support
set connection state ~atomically, preventing race condition in setup threads
have slurm schedule cores as consumable resources, not just memory
not quite finished support for fetching hostkeys from launched instances
sync with upstream/master
move this to a branch

Revert "not quite finished support for fetching hostkeys from launched instances"

This reverts commit 6b17dd7847b7a58e439311fd58cec08b7b053eb9.
calculate aws key fingerprints the same way aws does
calculate aws key fingerprints the same way aws does
slurmctld will get node core counts, memory amounts from the nodes
EC2: support customization of mounted root volume (size, storage type, iops)