B #3939: ldap search attributes should be an array

(cherry picked from commit 5d1e5f6a4c13d9302bf8e407caa0311a7f70c3e9)

src/authm_mad/remotes/ldap/ldap_auth.rb

@@ -196,7 +196,7 @@ class OpenNebula::LdapAuth
             filter = "(#{@options[:group_field]}=#{@user[@options[:user_field]].first})"
             @ldap.search(
                 :base       => @options[:base],
-                :attributes => "dn",
+                :attributes => [ "dn" ],
                 :filter     => filter
             ) do |entry|
                 if @mapping[entry.dn]