Add 'reload_certificate' flag to postcoh_finalsink.py

'reload_certificate' flag allows GraceDB to look for the latest X509 authentication certificate. This is especially useful during cluster maintenance.

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- self.gracedb_client = GraceDb(gracedb_service_url)

+ self.gracedb_client = GraceDb(gracedb_service_url, reload_certificate=True)

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