Centralized all LDAP config logic in to `Gitlab::LDAP::Config`. We had varying configuration for devise/omniauth and other things. For example, `user_filter` was never taken in to account for devise/omniauth so a user object would always be created, even if the user did not match the user_filter. 

Fixes gitlab-org/gitlab-ce#21195, https://gitlab.com/gitlab-org/gitlab-ce/issues/15396 and gitlab-org/gitlab-ce#13296

See merge request !6606