Fix Bug #10096

Add retry with incremental backoff to unmount() function in
'src/ceph-disk' to address bug #10096.

Under certain conditions, a umount instruction issued during 'ceph-disk
prepare' fails to unmount a temporary mount. Very short delays (even
those caused by a 'sync' command or strace information) have been able
to fix the issue. The root cause is unknown; however, retrying with an
incremental backoff is a reasonable workaround.