This also worked for me if you want to use shell environment variables:
ansible-playbook -i "localhost," ldap.yaml --extra-vars="LDAP_HOST={{ lookup('env', 'LDAP_HOST') }} clustername=mycluster env=dev LDAP_USERNAME={{ lookup('env', 'LDAP_USERNAME') }} LDAP_PASSWORD={{ lookup('env', 'LDAP_PASSWORD') }}"