The 'releasenotes' gate job does not run our tox section. This allowed a
releasenote to land that had a filename in an incorrect format because
it was not created with 'reno new'.
Move the running of 'tools/check-releasenotes.py' to the pep8 section
and whitelist the releasenote that got merged.
Change-Id: Id5bd8e0cbc9186c89f682224088a8f23998d59c3