Enhance the LDAP group synchronization

Merged username-removed-87381 requested to merge 1and1_ebusiness/gitlab-ee:master into master

The response of our company's LDAP for a group looks like this:

dn: <dn-of-the-selected-group>
cn: <name-of-the-selected-group>
submember: uid=<username-of-a-submember>
member: uid=<username-of-a-member>

So the current implementation of the LDAP group synchronization in GitLab can't find any registered user in such a group. This merge request will fix this.

This is a "copy" of this merge request https://gitlab.com/subscribers/gitlab-ee/merge_requests/27 that was in the wrong repository.