LDAP list groups with missing member entry

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,
and probably log a warning message about the missing user.

Fixes bug 1174585

Change-Id: Idf7c8c7f87affc4a72c5fe5e18e09a0f362e2646
Merge "LDAP list groups with missing member entry"