Use ldap3-python instead of ldap-python

ldap3-python is native python code and supports Python3.

Also fixed:

* share common ldap code in backend/base.py
* use logging module for debug messages.

Change-Id: I1ec90f3d144408189b32bd55b0313f44ea69c625
Add testing support to setup.py.

Change-Id: I3ce19c9b6b8977071b8b0f1cd41d2d0b234fce3e