Also accept `global_var` as a syntax for variable that will be used
for *all* the nodes in the cluster.
Add documentation for extra varibles that can be defined on gluster and slurm playbooks.
Squashed 'elasticluster/providers/ansible-playbooks/' changes from b4cec78..15493d4

15493d4 Make `SelectType` and `SelectTypeParameters` configurabile via variables `slurm_selecttype` and `slurm_selecttypeparameters`

git-subtree-dir: elasticluster/providers/ansible-playbooks
git-subtree-split: 15493d470d4f774ba6780776b1c291809e158723
Update ansible playbooks
Add configuration option `zone` for google cloud section.