[helm] bump helm version to 2.15.1

[jmlrt]

does what it says in the tin

Had to bump directly to v2.15.1 which contain an emergency fix we need in our test scripts (helm/helm#6722).

• Helm 2.15.0 changelog: https://github.com/helm/helm/releases/tag/v2.15.0

• Helm 2.15.1 changelog: https://github.com/helm/helm/releases/tag/v2.15.1

• Chart version not bumped (the versions are all bumped and released at the same time)

• README.md updated with any new values or changes

• Updated template tests in ${CHART}/tests/*.py

• Updated integration tests in ${CHART}/examples/*/test/goss.yaml

[jmlrt]

Elasticsearch chart lint is failing with Helm 2.15.0:

10:22:33 helm lint --strict ./
10:22:33 Error: 1 chart(s) linted, 1 chart(s) failed
10:22:33 ==> Linting ./
10:22:33 [ERROR] Chart.yaml: apiVersion is required
10:22:33 [ERROR] templates/: render error in "elasticsearch/templates/service.yaml": template: elasticsearch/templates/service.yaml:24:14: executing "elasticsearch/templates/service.yaml" at <.Values.service.nodePort>: map has no entry for key "nodePort"

elastic+helm-charts+pull-request+template-testing/227/CHART=elasticsearch

[jmlrt]

jenkins test this please

[Crazybus]

LGTM!