vol_clone_wipe: Fix dependence of scrub

1) 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.

2) Skip related test if libvirt don't support option --prealloc-metadata.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #283 from Hao-Liu/fix_vol_wipe

vol_clone_wipe: Fix dependence of scrub