fix: rename the secret to access the artifact repository (#141)

4b57f05 renamed the Secret argo-controller applies to configure the artifactRepository.
This change should be reverted back to mlpipeline-minio-artifact as pipelines depend on
this Secret to be named exactly like that.

The places where this Secret is expected are:
* [kfp-api](https://github.com/kubeflow/manifests/blob/master/apps/pipeline/upstream/base/pipeline/ml-pipeline-apiserver-deployment.yaml#L69)
* [kfp-ui](https://github.com/kubeflow/manifests/blob/master/apps/pipeline/upstream/base/pipeline/ml-pipeline-ui-deployment.yaml#L42)

Fixes canonical/kfp-operators/issues/345

charms/argo-controller/src/charm.py

@@ -155,7 +155,7 @@ def _context_callable(self):
             "secret_key": b64encode(
                 self.object_storage_relation.component.get_data()["secret-key"].encode("utf-8")
             ).decode("utf-8"),
-            "mlpipeline_minio_artifact_secret": "mlpipeline-minio-artifact-secret",
+            "mlpipeline_minio_artifact_secret": "mlpipeline-minio-artifact",
             "argo_controller_configmap": ARGO_CONTROLLER_CONFIGMAP,
             "s3_bucket": self.model.config["bucket"],
             "s3_minio_endpoint": (