set better defaults for ldap

* LDAP params is a new field. It contains the kwargs that will be passed
to the python class specified by group type. The default for group type
is MemberDNGroupType. The required params are now those in the defaults.

awx/sso/conf.py

@@ -306,6 +306,7 @@ def _register_ldap(append=None):
         category=_('LDAP'),
         category_slug='ldap',
         default=collections.OrderedDict([
+            ('member_attr', 'member'),
             ('name_attr', 'cn'),
         ]),
         placeholder=collections.OrderedDict([