Fix playbook-local host_vars when running from .

Since ansible 1.2, it became possible to place a host_vars
directory in the same directory as a playbook, making it possible
to keep host_vars local to that playbook there. However, due to
python's os.path.dirname, a action such as:

 $ ansible-playbook pb.yml

..would not pick up the host_vars as os.path.dirname("pb.yml")
returns "", unlike the unix command dirname that would return
".". Substituting "pb.yml" on the command line with "./pb.yml"
would do the trick, but is not always intuitive. This patch
solves the problem until python solves issue18547 [1].

[1] http://bugs.python.org/issue18547