Ensure namespace exists in Kubernetes cluster

This fixes https://gitlab.com/gitlab-org/gitlab/issues/118598.

changelogs/unreleased/managed-cluster-applications-ci-template-ensure-namespace.yml

+---
+title: Ensure namespace is present for Managed-Cluster-Applications CI template
+merge_request: 21903
+author:
+type: fixed

lib/gitlab/ci/templates/Managed-Cluster-Applications.gitlab-ci.yml

@@ -9,6 +9,7 @@ apply:
     INGRESS_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/ingress/values.yaml
     SENTRY_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/sentry/values.yaml
   script:
+    - kubectl get namespace "$TILLER_NAMESPACE" || kubectl create namespace "$TILLER_NAMESPACE"
     - gitlab-managed-apps /usr/local/share/gitlab-managed-apps/helmfile.yaml
   only:
     refs: