logdir is not respected in Scenario.defaults

[goodboy]

Thanks to @fchareyron for finding this in #19.
It seems we never respect the logdir arg passed into pysipp.walk() or pysipp.scenario() such that it's never set in pysipp.Scenario.defaults. It is however overridden in the underlying agents (by original design) but this is arguably not clear to the user.

In other words:

>>> next(pysipp.walk('./scens/', logdir='/var/tmp'))[1].logdir
'/tmp'

@fchareyron's change to pysipp/__init__.py:96:

- walk(dirpath, autolocalsocks=autolocalsocks, **scenkwargs)
+ walk(dirpath, logdir=logdir, autolocalsocks=autolocalsocks, **scenkwargs)

does resolve this at the Agent level for pysipp.scenario as per:

In [7]: s = pysipp.scenario('./tests/scens/default/', logdir='/var/tmp')

In [8]: s.agents
Out[8]: 
OrderedDict([('uas', <pysipp.agent.UserAgent at 0x7f1b40cc0ed0>),
             ('uac', <pysipp.agent.UserAgent at 0x7f1b40cc0e90>)])

In [9]: s.agents['uac'].logdir
Out[9]: '/var/tmp'

but logdir should really be assigned in Scenario.defaults as well.
We need a test and fix for this.