Velero was including DownwardAPI volumes when backing up with restic.
When restoring these volumes, it triggered a known issue with restic (as
seen in #3863). Like projected volumes, these volumes should be skipped
as their contents are populated by the Kubernetes API server.

With this change, we are now skipping the restic backup of volumes with
a DownwardAPI source. We are also skipping the restore of any volume
that had a DownwardAPI source as there will exist backups that were
taken prior to this fix being introduced. This will allow these backups
to be restored succesfully.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>