Merge branch 'sh-backport-ee-logger-changes' into 'master'

Backport logger changes from EE

See merge request gitlab-org/gitlab-ce!20621

lib/gitlab/logger.rb

@@ -4,10 +4,18 @@ module Gitlab
       file_name_noext + '.log'
     end
 
+    def self.debug(message)
+      build.debug(message)
+    end
+
     def self.error(message)
       build.error(message)
     end
 
+    def self.warn(message)
+      build.warn(message)
+    end
+
     def self.info(message)
       build.info(message)
     end