Fix race condition in RemoteFS create_snapshot_online

Fix race condition in RemoteFS create_snapshot_online

There's a `while True` loop in create_snapshot_online and each
iteration can make the `cinder-volume` process sleep up to 10sec. In
the mean time, if Nova fails to create the snapshot, the snapshot
status could turn to "error" and someone could want to delete the
erroneous snapshot, which would make its status to be "deleting".

In that case the `while True` loop would never exit. Cinder consumes
100% of CPU and keeps logging "Status of snapshot XX is now deleting".

The patch fix this issue by exiting the `while True` loop if we detect
that the snapshot is to be deleted.

Closes-Bug: #1538496

Change-Id: I5de0e8479a552ce101cecd06a874a170e54d5c18