the --kubernetes-services flag doesn't work
I'm trying to register a kubernetes runner and use the new `--kubernetes-services <value>` flag to add a `[[runners.kubernetes.services]]` section but the command is generating an incorrect `config.toml` file.
```bash
gitlab-runner register --kubernetes-services "docker:dind"
```
results in:
```toml
# ...
[[runners.kubernetes.services]]
name = ""
```
issue