vol_clone_wipe: Fix dependence of scrub

Using an algorithm other than `zero` when doing `vol-wipe` needs `scrub`
package.

This fix check the existence of `scrub`. If fails, it logs a warning
message and using `zero` algorithm instead.

Signed-off-by: Hao Liu <hliu@redhat.com>
Yes, `valid_algorithms` is a list. but `wipe_algorithms` is a string parsed from cfg file.