RFR: 1919: Make it possible to disable the "/clean" command per repository

[Zhao Song]

This patch is trying to make it possible to configure the "/clean" command to be disabled on a repository.

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Commit messages:
 - update factory test
 - SKARA-1919

Changes: https://git.openjdk.org/skara/pull/1629/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1629&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1919
  Stats: 101 lines in 6 files changed: 94 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1629.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1629/head:pull/1629

PR: https://git.openjdk.org/skara/pull/1629