loader-metadata: Name appropriately the container

Previously the chart refused to install:

```
one or more objects failed to apply, reason: Deployment.apps "loader-metadata" is
invalid: [spec.template.spec.containers[0].name: Invalid value: "loader_metadata": a
lowercase RFC 1123 label must consist of lower case alphanumeric characters or '-', and
must start and end with an alphanumeric character (e.g. 'my-name', or '123-abc', regex
used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?'),
```

Refs. swh/infra/sysadm-environment#4794