With a slow file system, umount can return 0 and the immediately
following remove can fail with a "Device or resource busy" error.
This happened in DevStack in disk-image-create where unmount_image
is followed by an immediate cleanup_build_dir.
Solution is to apply same logic from bug 1332521 to allow the
remove to retry on failure (up to 5s) in case the umount has not
completed.
Change-Id: I3337e2b4ad0111e77f79dc179439cdfea8ebdeda
Closes-Bug: #1527721