xtrabackup --copy-back must take plugin-load setting from my.cnf

During the backup xtrabackup is saving plugin-load setting from the live server to the backup-my.cnf. It then will use it for prepare if needed. Currently, setting from backup-my.cnf is also used for -copy-back. However, if user wants to migrate from one plugin to another, he has no option but to edit backup-my.cnf. This also applies to SST when different nodes using different plugins. xtrabackup -copy-back must read plugin-load and early-plugin-load settings from my.cnf.