Previously, only the rook-ceph operator chart was linted in the CI. With this change, the rook-ceph-cluster chart is linted additionally. for developer convenience, The linting is realized via a new make target 'helm.lint'that can easily be invoked locally if the chart-testing tool 'ct' is available. Fixes: #15966 Signed-off-by: Michael Adam <obnox@samba.org>