Merge `deploy_vms_standalone` and `deploy_vms_cluster`

[eroussy]

Abstract
Currently, two playbooks (and associated roles) are used to deploy VMs depending on the setup (cluster or standalone)
vm-manager is capable of handling cluster or standalone setup, so it should be used into both.
The two playbooks (and roles) should be merged in one deploy_vms that handles both cases.

This also corrects #672

Detailed Description
The deploy_vms_cluster role uses vm-manager create behavior, whereas the deploy_vms_standalone one uses Libvirt directly.
vm-manager should be able to create a VM in standalone mode also. I suggest deleting the standalone playbook and adapting the cluster playbook to work in both cases.

Note that the deploy_vms_cluster playbook uses the cluster_vm library (library/cluster_vm.py) that calls vm-manager.
Even though vm-manager is capable handling standalone, cluster_vm may not be. It may require additional adaptation

Second note, this change will create an API break by renaming the playbooks. We must either

• Document it, if done before the 2.0 (API break is fine)
• Keep the old playbooks name and link them to the new mechanism (to avoid creating an API break)

Work to be done

• Test deploy_vms_cluster in standalone
• Adapt if necessary (it must work seamlessly in standalone or cluster mode)
• Rename it into deploy_vms
• Remove deploys_vms_standalone
• Handle the API break (documentation, or mitigation)
• Update the wiki (multiple references to deploy_vms_standalone or cluster)