use existing django_auth_ldap config values where possible

src/memberdb/account_backend.py

@@ -29,8 +29,8 @@ log = logging.getLogger('ldap')
 ldap_uri = getattr(settings, 'AUTH_LDAP_SERVER_URI')
 ldap_user_dn = getattr(settings, 'LDAP_USER_SEARCH_DN')
 ldap_base_dn = getattr(settings, 'LDAP_BASE_DN')
-ldap_bind_dn = getattr(settings, 'LDAP_BIND_DN')
-ldap_bind_secret = getattr(settings, 'LDAP_BIND_SECRET')
+ldap_bind_dn = getattr(settings, 'AUTH_LDAP_BIND_DN')
+ldap_bind_secret = getattr(settings, 'AUTH_LDAP_BIND_PASSWORD')
 make_home_cmd = ["sudo", "/services/uccportal/src/memberdb/root_actions.py"] 
 make_mail_cmd = 'ssh -i %s root@mooneye "/usr/local/mailman/bin/add_members" -r- ucc-announce <<< %s@ucc.asn.au'
 make_mail_key = './mooneye.key'