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>