The checks performed by this script aren't always useful to downstream
consumers of the repo so allow them to disable the script without having
to make changes to tox.ini.
NOTE(lyarwood): This backport has
Ie8a672fd21184c810bfe9c0e3a49582189bf2111 squashed into it to ensure the
introduced env var is passed into the pep8 tox env.
tox: Add passenv DISABLE_CHERRY_PICK_CHECK to pep8
I4f551dc4b57905cab8aa005c5680223ad1b57639 introduced the environment
variable to disable the check-cherry-pick.sh script but forgot to allow
it to be passed into the pep8 tox env.
Change-Id: I4f551dc4b57905cab8aa005c5680223ad1b57639
(cherry picked from commit 610396f8ad5fe8c3abb7731fcd42c81e5246a938)
(cherry picked from commit 3d86df068a7a78f8faa9ac6437ff4fc19f5cebcc)
(cherry picked from commit cb96119e59df64d6884431c8f2e8f6ced8318bd7)
3c8841595a