Fixed broken tests for pkg/networkfs/etchosts

The tests for `pkg/networkfs/etchosts` don't build properly:

```
+ go test github.com/dotcloud/docker/pkg/networkfs/etchosts
# github.com/dotcloud/docker/pkg/networkfs/etchosts
./etchosts_test.go:17: not enough arguments in call to Build
./etchosts_test.go:39: not enough arguments in call to Build
./etchosts_test.go:61: not enough arguments in call to Build
FAIL    github.com/dotcloud/docker/pkg/networkfs/etchosts [build failed]

Tests failed: ./pkg/networkfs/etchosts
```

This pull request fixes the above issue, which was caused by a change to the `pkg/networkfs/etchosts` `Build` signature in 53f38a14 and the tests not being updated.

/cc @vieux