SG support import remote full backup

[yinweiishere]

To support business non-stop migration, step2:
2.1 transit the full backup storage to second site;
2.2 tell SG the location of the backup storage (ensure accessible)

Here 2.2 means, SG should provide a way to import a specified storage as a specified backup.
In actual fact, it's sort of remote backup, but the full data is transit by truck instead by network, where we make the backup registration work as a separate step.