Action to restore database from the backup is missing

[Tytus Kurek]

Currently the percona-cluster charm supports a "backup" action which can be used to take a backup of the running databases. However, it is missing an option to restore the databases from the backup which is weird. The databases can be restored manually by running the following commands:

innobackupex --apply-log /var/backups/mysql/<BACKUP_DIR>
systemctl stop mysql
mv /var/lib/percona-xtradb-cluster /var/lib/percona-xtradb-cluster_OLD
mkdir /var/lib/percona-xtradb-cluster
innobackupex --copy-back /var/backups/mysql/<BACKUP_DIR>
rsync --ignore-existing /var/lib/percona-xtradb-cluster_OLD/* /var/lib/percona-xtradb-cluster/
chown -R mysql:mysql /var/lib/percona-xtradb-cluster
chown root:root /var/lib/percona-xtradb-cluster/seeded
systemctl start mysql

I think the consistent solution should have both "backup" and "restore" actions implemented.