Support Vagrant for local testing.

Adds a Vagrantfile supporting just one VM (Debian 6), provisioning the
`test.yml` playbook, which prompted the creation of `inventory` directory
to hold Travis and Vagrant inventories (Which could, as well, be in the
same file).

Moves contents off `tests` directory:
    - `tests/test.yml` -> `test.yml`;
    - `tests/inventory` -> `inventory/travis.ini`;

Changes `test.yml` from running the role installed to including the
tasks, variable files and handlers provided by the role, `.travis.yml`
was changed to reflect all these changes.