Pull in ldap dependencies via ldappool

For the transition from pyldap to python-ldap, we can use ldappool
to pull in the correct variant of ldap for us. so we don't need
to depend on pyldap nor python-ldap.

This patch was previously merged but pyldap was mistakenly pulled
back in [1].

[1]: https://review.openstack.org/#/c/573508/

Change-Id: I55456ea886de950cd48981b1117aa08b38e4a2ba
Co-Authored-By: Dirk Mueller <dirk@dmllr.de>

defaults/main.yml

@@ -425,7 +425,6 @@ keystone_pip_packages:
   - keystone
   - ldappool
   - osprofiler
-  - pyldap
   - PyMySQL
   - python-memcached
   - python-openstackclient