nspawn.py: Fix bad keyword assignment
nspawn.py: Fix bad keyword assignment

Currently every keyword for _pull_image is considered a bad keyword.

```
% salt node nspawn.pull_tar url bar
ERROR executing 'nspawn.pull_tar': The following invalid keyword arguments were passed: verify=False.
```

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