Replaced 'localhost' to '127.0.0.1' in mysql_clientdb_plugin.inc.php plugin

The reason is that 127.0.0.1 is much more universal in use. For example, to mysql servers configured with ''skip-name-resolve'' is not able to connect when is only 'localhost' configured for mysql users. Also in chrooted environment (FPM chroot) is much more practical and universal to use 127.0.0.1 rather than 'localhost'. Vice versa connection works properly (connecting to host 'localhost' when 127.0.0.1 is allowed for user - depends on hosts file, but in chroot it is not necessary to copy hosts file into chroot).