Add API to wait for backup completion in integration tests.
Add API to wait for backup completion in integration tests.

Signed-off-by: Rohit-PX <83d5e1e49bd5f0ebbf6c9ba40416057fac1b5d76@portworx.com>

libopenstorage/stork,libopenstorage/stork,libopenstorage/stork