Adding Bigtable system tests to tox environment (for Py2.7 only).

Author: dhermes

tox.ini

@@ -98,6 +98,13 @@ basepython =
     python2.7
 commands =
     python {toxinidir}/system_tests/attempt_system_tests.py
+    python {toxinidir}/system_tests/run_system_test.py --package=bigtable
+    python {toxinidir}/system_tests/run_system_test.py --package=bigtable-happybase
+setenv =
+    PYTHONPATH =
+deps =
+    {[testenv]deps}
+    grpcio >= 0.13.0
 passenv = GOOGLE_* GCLOUD_* TRAVIS* encrypted_*
 
 [testenv:system-tests3]