Ansible Provisioner: 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, a String is still accepted (but for a single argument, of course).

Note: Due to low/expert usage of this option, I think that it is (for now) not necessary to add more robust validation on this parameter (e.g. Array of String type checking or argument syntax pattern matching). Even though, it is maybe a good idea to address these validation questions, but I would like to treat in a separate issue, with a more global perspective (all options of Ansible provisioner, comparing/unifying with other provisioners, etc.)

@mitchellh please let me know, if this fix is good enough. Or if I should word/implement it differently...