test_storwize_vdisk_copy_ops fails if green thread context switch

There is loopingcall(green thread)will cause _rm_vdisk_copy_op() run
twice.On the second time, the exception KeyError is not caught.

Closes-Bug: #1302670

Change-Id: I788f07c33f76693a13f0d581e687a32c6736a394
Merge "test_storwize_vdisk_copy_ops fails if green thread context switch"