Provide backup/restore actions

[Andrea Ieri]

There is currently no charmed way to backup/restore an easyrsa installation.
Stateful data lives in /var/lib/juju/agents/unit-<unit name>/charm/EasyRSA/pki but I'm not sure if it's just a matter of saving and restoring that directory.

[Martin Kalcok (martin-kalcok)]

I would approach this by creating 4 actions for easyrsa charm:

* backup
* restore name=<backup_file_name>
* list-backups
* delete-backup name=<backup_file_name> [all=no]

Caveat: From what I gathered, the `juju run-action` is not designed to transport files between local and target system, so these action will need to be used in conjunction with `juju scp`.

Backup action:
This action creates snapshot of pki and stores it in predefined location (/home/ubuntu/easyrsa_backups/) into the timestamped archive (easyrsa-backup-YYYYMMDD-HHMMSS.tar.gz). The archive will contain 'pki' directory from $CHARM_DIR/EasyRSA/ which should contain all the required data needed for backup. I'm currently investigating whether also $CHARM_DIR/.unit-state.db needs to be backed up or if it can be restored purely based on data from pki directory.
When the backup is created, output of the action will contain 'juju scp' command that user can use to copy the backup file to the local machine. Similar to how debug action works in https://github.com/juju-solutions/layer-debug

Restore action:
This action takes one parameter 'name', looks for the backup file with that name in the backup directory and restores the easyrsa pki.
If user wants to restore pki from the backup file located on the local machine. It will have to be uploaded to the '/home/ubuntu/easyrsa_backups' first, using 'juju scp'.

List action:
Lists all the files in the backup directory

Delete action:
This action takes either parameter 'name' and attempts to delete file with that name from the backup folder, or if parameter 'all=yes' is used, all backups in the directory will be deleted.

[Alvaro Uria (aluria)]

PR is ready for review at https://github.com/charmed-kubernetes/layer-easyrsa/pull/30