RFR: prbot: Allow overriding jcheck configuration

[Robin Westberg]

Hi all,

Please review this change that allows the pull request bot to use a jcheck configuration file from a separate
repository, overriding the one (if any) present in the checked repository.

Best regards,
Robin

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Commit messages:
 - prbot: Allow overriding jcheck configuration

Changes: https://git.openjdk.java.net/skara/pull/755/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=755&range=00
  Stats: 212 lines in 13 files changed: 180 ins; 10 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/755.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/755/head:pull/755

PR: https://git.openjdk.java.net/skara/pull/755