Glance backup: Omit directory timestamps

Rsync is throwing an error about failing to set times
on "/srv/glance/images/." I don't know why it can't set
them, but adding -O stops it from trying, and we don't
need it.

Bug: T135463
Change-Id: I9a0687c7dd456a0d2a621ae60b4ab482a8b9a8bf