infra: unset region during integ tests (#1308)

Author: laurenyu

buildspec.yml

@@ -16,7 +16,7 @@ phases:
 
       - start_time=`date +%s`
       - |
-        execute-command-if-has-matching-changes "tox -e py36 -- tests/integ -m \"not local_mode\" -n 512 --reruns 3 --reruns-delay 5 --durations 50 --boto-config '{\"region_name\": \"us-east-2\"}'" "tests/" "src/*.py" "setup.py" "setup.cfg" "buildspec.yml"
+        execute-command-if-has-matching-changes "env -u AWS_DEFAULT_REGION tox -e py36 -- tests/integ -m \"not local_mode\" -n 512 --reruns 3 --reruns-delay 5 --durations 50 --boto-config '{\"region_name\": \"us-east-2\"}'" "tests/" "src/*.py" "setup.py" "setup.cfg" "buildspec.yml"
       - ./ci-scripts/displaytime.sh 'py36 tests/integ' $start_time
 
   post_build: