Merge branch 'backup-database-timeout-fix' into 'master'

Fix timeout issue for rake task gitlab:backup:create

This is a fix for a database timeout which can occur when the backup
create task is taking very long (1-2 hours).
It seems that ActiveRecord is loosing the connection after a hour
idleness and need to be reconnected before use.

See merge request !2757

lib/backup/manager.rb

 module Backup
   class Manager
     def pack
+      # Make sure there is a connection
+      ActiveRecord::Base.connection.reconnect!
+
       # saving additional informations
       s = {}
       s[:db_version]         = "#{ActiveRecord::Migrator.current_version}"