Ansible: Improvements for 'extra_vars' argument

`extra_vars` argument can now:
- contains a hierarchical set of parameters (instead of flat Key-Value Pairs)
- alternatively refers to a configuration file (instead of embedded Hash)

See Ansible Documentation for version requirement details:
http://www.ansibleworks.com/docs/playbooks_variables.html#passing-variables-on-the-command-line
Merge pull request #2366 from gildegoma/ansible-extra-vars

provisioners/ansible: Improvements for 'extra_vars' argument