Application namespaces should be created only by the controller since the ApplicationGroup owns each of the created resources

Currently, the namespace creation is duplicated in both the reconciler and the executor container. It would be better to have the reconciler create the namespace while ignoring 'Already Exists' errors.

Additionally, in keeping with helm client's `--create-namespace` option, the created namespace should also be labeled with
```yaml
labels:
name:
```