Ignore return code in zfs grains
Ignore return code in zfs grains

When there are no pools zpool list exists with 1.
It's possible and OK to have zfs but no have any pools.

```
[ERROR   ] Command '/usr/sbin/zpool list -H -o name,size' failed with return code: 1
[ERROR   ] output:
```

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt