provisioners/ansible: add first unit tests
provisioners/ansible: improve unit tests (wip)

- Don't mock the config object, but use a true instance
- When possible, take advantage of Rpsec before/after hooks to
  reduce code repetitions
- Add an example ("with inventory_path option")
- Use a global variable to store the path of the generated inventory
- Miscellaneous changes in existing examples (style, fixes)
provisioners/ansible: add more unit tests

Remove wrong usage of shared examples and introduce embedded class methods
(as kind of simple "RSpec macros") to reduce code duplication.
provisioners/ansible: update specs after [GH-3436]
provisioners/ansible: use strict boolean options

With this change, the ansible provisioner fully complies with
the current user documentation.
Merge pull request #3287 from gildegoma/ansible-unit-tests

provisioners/ansible: add first unit tests