Set username field of users created from LDAP to uid attribute.

The previous approach of making username from user part of
email address (part of address before '@') has the following limitations:
* username in email is not guaranteed to be unique,
* email could have nothing to do with user identity in LDAP
  (like presonal email on an external email service).
On the other hand, LDAP uid is guaranteed to be unique,
as it represents user's login name on Unix/Linux systems.

To understand this change, please note that gitlab_omniauth-ldap
Ruby gem uses the following mapping to store LDAP attrinbutes in auth info:
* dn attribute of LDAP entry is stored in uid attribute of auth info;
* uid attribute of LDAP entry is stored in nickname attribute of auth info.
Signed-off-by: Oleg Girko <oleg.girko@jolla.com>

lib/gitlab/ldap/user.rb

@@ -13,8 +13,8 @@ module Gitlab
         def find_or_create(auth)
           @auth = auth
 
-          if uid.blank? || email.blank?
-            raise_error("Account must provide an uid and email address")
+          if uid.blank? || email.blank? || username.blank?
+            raise_error("Account must provide a dn, uid and email address")
           end
 
           user = find(auth)
@@ -64,6 +64,10 @@ module Gitlab
           model.where(provider: provider, extern_uid: uid).last
         end
 
+        def username
+          auth.info.nickname.to_s.force_encoding("utf-8")
+        end
+
         def provider
           'ldap'
         end