Fixes for parameter scan samples

[peruzzim]

PR description:

Two fixes for samples with parameter scan feature:

• properly merge per-point normalization across files in central production
• remove slashes from model name to fix #503

[gpetruc-bot]

Automatic test started, see https://gitlab.cern.ch/cms-nanoAOD/nanoAOD-integration/pipelines/1433277/builds