We ran into a lot of problems using a finalizer on the backup to allow
the Ark server to clean up all associated backup data when deleting a
backup.

Users also found it less than desirable that deleting the heptio-ark
namespace resulted in all the backup data being deleted.

This removes the finalizer and replaces it with an explicit
DeleteBackupRequest that is created as a means of requesting the
deletion of a backup and all its associated data. This is what `ark
backup delete` does.

If you use kubectl to delete a backup or to delete the heptio-ark
namespace, this no longer deletes associated backups. Additionally, as
long as the heptio-ark namespace still exists, the Ark server's
BackupSyncController will continually sync backups into the heptio-ark
namespace from object storage.
Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>