WIP: fakeldap.py: Don't rely on KeyError exceptions when accessing non-exi…

[sunweaver]

…stent keys of MockLDAP.directory.

The implementation of the internal MockLDAP.directory dictionary is
defined via
.
self.directory = defaultdict(lambda: {})
.
This results in a dictionary that always returns an empty dict for any
key requested:
.
>>> print (self.directory['some_key'])
{}
.
Thus, we cannot rely on the raisal of KeyError exceptions when accessing
a non-existing LDAP DNs in the MockLDAP.directory.
.
Instead, we check if MockLDAP.directory['some_key'] returns an empty
dict {} value.

[sunweaver]

Please wait with reviewing this PR until #21 has been finally merged.