Fix a flaw in smoketests CI (#475)

* Run artman smoketests in circleci

* Only execute smoketests using circleci on master branch

* Specify additional env var so that the artman docker container that runs the smoketests knows to use the cached gapic yamls instead of the latest ones. This will match the behavior of .

.circleci/config.yml

@@ -22,6 +22,7 @@ jobs:
           name: Run smoke tests
           command: |
             mkdir /tmp/reports
+            export RUNNING_IN_ARTMAN_DOCKER=True
             smoketest_artman.py --root-dir=/var/code/googleapis/ --log=/tmp/reports/smoketest.log
       - store_test_results:
           path: /tmp/reports