new variable so we can chose attribute used for DN

tasks/ldap_user_inc.yml

@@ -4,7 +4,7 @@
       firstname: "{{ item.firstname }}"
       lastname: "{{ item.lastname }}"
       email: "{{ item.email }}"
-      dn: "uid={{ item.firstname |lower }}, ou={{ org }},{{ ldap_base }}"
+      dn: "uid={{ item[ nlu_dn_attribute ] |lower }},ou={{ org }},{{ ldap_base }}"
       password: "{{ (item.password=='')| ternary(lookup('community.general.random_string', min_lower=1, min_upper=1, special=false,min_numeric=1, length=14), item.password ) }}"
 
   - name: "add ldap account {{dn }}"