tools: Allow check-cherry-picks.sh to be disabled by an env var

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 610396f)

Author: lyarwood

tools/check-cherry-picks.sh

@@ -4,6 +4,11 @@
 # to verify that they're all on either master or stable/ branches
 #
 
+# Allow this script to be disabled by a simple env var
+if [ ${DISABLE_CHERRY_PICK_CHECK:-0} -eq 1 ]; then
+    exit 0
+fi
+
 commit_hash=""
 
 # Check if the patch is a merge patch by counting the number of parents.

tox.ini

@@ -49,6 +49,8 @@ commands =
 description =
   Run style checks.
 envdir = {toxworkdir}/shared
+passenv =
+  DISABLE_CHERRY_PICK_CHECK
 commands =
   {[testenv:mypy]commands}
   bash tools/flake8wrap.sh {posargs}