No bug. Use system python with developer_config instead of /tools/python27. r=NPOTB

--HG--
extra : commitid : DcU1pBPX27S
extra : rebase_source : b849a3b85b51834537ec50ceb1b2a4dfd2fbade1

testing/mozharness/configs/developer_config.py

@@ -25,6 +25,7 @@ config = {
     "exes": {
         "hgtool.py": os.path.join(LOCAL_WORKDIR, "hgtool.py"),
         "gittool.py": os.path.join(LOCAL_WORKDIR, "gittool.py"),
+        "virtualenv": ["python", "-m", "virtualenv"],
     },
     "env": {
         "PIP_TRUSTED_HOST": "pypi.pub.build.mozilla.org",