LDAP list group users not fail if user entry deleted

Using the LDAP identity backend,
if a group member entry doesn't exist in the LDAP server anymore
and the group's members are listed using GET /v3/groups/{groupId}/users,
Keystone returns 404 Not Found.

The server should return all the group members that do exist
and ignore the missing members.

Fixes bug 1174585

Change-Id: I97b53e3d5a5810aa0818b785e23a1948499b29e8