Ansible: Fix a bug in `raw_arguments` option

Without this change, it is not possible to pass more than one "raw"
argument, which was not the expected behavior. In addition to Array
format, String (for a single argument) is still accepted (for sake of
"backward compatibility" and ease of use).

Note: Due to low/expert usage of this option, I think that it is not
necessary to add more robust validation on this parameter (e.g. Array
of String type checking or argument syntax pattern matching). Use it at
your own risk ;-)
Merge pull request #2667 from gildegoma/bugfix_ansible_raw_args

provisioner/ansible: Fix a bug in `raw_arguments` option