ldap: use python3-ldap instead of python-ldap

python-ldap needs a c library
while python3-ldap is pure python

pure python is prefered for simplification of deployment with pip
(no need to apt-get builddeps, works better with paas)

unlike its name is suggesting, it support python 2

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>