Purge the amanda-server packages/configurations

Purge the packages and configurations
Remove the now defunct collection of files tagged backup_host
Absent the vtape creation script

When merging this, don't forget to remove from the private repo:

 * files/backup/ssh-keys/authorized_keys_server
 * files/backup/ssh-keys/amanda_restore

Change-Id: Ia417514f135e8d90361659adf877eef7d16a61f6