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]
When there are multiple versions of a box are available it should be checked if an update is available for the highest version.

Example:

There are version 0.2.10 and 0.2.11 available. The latest update is version 0.2.12.

```
berendt/ubuntu-14.04-amd64          (virtualbox, 0.2.10)
berendt/ubuntu-14.04-amd64          (virtualbox, 0.2.11)
```

Without this patch it will be checked if a higher version than 0.2.10 is available.

```
Checking for updates to 'berendt/ubuntu-14.04-amd64'
Version constraints: > 0.2.10
Provider: virtualbox
Updating 'berendt/ubuntu-14.04-amd64' with provider 'virtualbox' from version
'0.2.10' to '0.2.12'...
Loading metadata for box 'https://vagrantcloud.com/berendt/ubuntu-14.04-amd64'
Adding box 'berendt/ubuntu-14.04-amd64' (v0.2.12) for provider: virtualbox
Downloading: https://vagrantcloud.com/berendt/ubuntu-14.04-amd64/version/23/provider/virtualbox.box
Successfully added box 'berendt/ubuntu-14.04-amd64' (v0.2.12) for 'virtualbox'!
```

with this patch it will be checked if a higher version than 0.2.11 is available (the expected behaviour).

```
Checking for updates to 'berendt/ubuntu-14.04-amd64'
Version constraints: > 0.2.11
Provider: virtualbox
Updating 'berendt/ubuntu-14.04-amd64' with provider 'virtualbox' from version
'0.2.11' to '0.2.12'...
Loading metadata for box 'https://vagrantcloud.com/berendt/ubuntu-14.04-amd64'
Adding box 'berendt/ubuntu-14.04-amd64' (v0.2.12) for provider: virtualbox
Downloading: https://vagrantcloud.com/berendt/ubuntu-14.04-amd64/version/23/provider/virtualbox.box
Successfully added box 'berendt/ubuntu-14.04-amd64' (v0.2.12) for 'virtualbox'!
```
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
Merge pull request #3452 from berendt/box_update_sort_versions

command/box/update: update from latest version
Update CHANGELOG
commands/box/repackage: works with new stuff [GH-3372]
Update CHANGELOG
commands/box/repackage: better error if box not found
Fix tests