Ansible: Re-enable ControlPersist defaults when ANSIBLE_SSH_ARGS is used

Solve problem discussed in [GH-2952]
provisioners/ansible: add new option raw_ssh_args

Since the Ansible provisioner now potentially exports ANSIBLE_SSH_ARGS
variable, it is fair to allow to extend the content of this environment
variable (`ssh_args` parameters from ansible.cfg file have lower
priority)
Merge pull request #3088 from gildegoma/ansible-ssh-args

provisioner/ansible: Improve management of ANSIBLE_SSH_ARGS