Check dco_signoffs files for missing DCO hashes
When checking for missing DCOs, any hashes found to be documented in
the dco_signoffs (or a user-specified alternative) directory will no
longer be included in the missing list.
Some formatting improvements have also been made to the relevant
files, particularly removing unnecessary #noqa and #nosec directives,
which were not affecting pylint scores.
Issue: RELENG-3396
Change-Id: I7cdac03fb406c6ce862b997c01fb0fab7cfecd76
Signed-off-by: Eric Ball <eball@linuxfoundation.org>