add ldap group type like posixGroupType

* Adds pattern to easy add django-auth-ldap group types classes and to
pass parameters via AUTH_LDAP_GROUP_TYPE_PARAMS
* Adds new group type PosixUIDGroupType that accepts the attribute,
ldap_group_user_attr, on which to search for the user(s) in the group.

awx/sso/backends.py

@@ -42,6 +42,7 @@ class LDAPSettings(BaseLDAPSettings):
     defaults = dict(BaseLDAPSettings.defaults.items() + {
         'ORGANIZATION_MAP': {},
         'TEAM_MAP': {},
+        'GROUP_TYPE_PARAMS': {},
     }.items())
 
     def __init__(self, prefix='AUTH_LDAP_', defaults={}):