Use pipe between ceph backup diff export/import

We now use a piped transfer between the rbd export-diff
and import-diff for incremental backups/restores as
opposed to holding the entire diff in memory.

Change-Id: I33476d9b3934781413af5cd2867a11d825a5d78e
Fixes: bug 1244464
(cherry picked from commit d384d28e1c2c7c3ce0bd676fb1c2bdb8a2d98a03)
Merge "Use pipe between ceph backup diff export/import" into stable/havana