Rrefactor LDAP logins to use Users::CreateService

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13846:

I think using services over hooks makes sense; that's the preferred pattern in GitLab, and allows for selective skipping of some hooks etc. Did you find out why we are calling Users::UpdateService instead of Users::CreateService at all if the user doesn't exist yet? It seems to me like we should call the right service, not teach UpdateService about creating users :)