configmap e2e: use labels and label selector

[roycaihw]

The Bound Service Account Tokens feature creates a configmap kube-root-ca.crt in every namespace. Use a label to select the test objects we create.

/hold
wait for the tests to pass

[k8s-ci-robot]

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[yliaog]

/lgtm

[roycaihw]

https://travis-ci.org/github/kubernetes-client/python-base/jobs/752929229

=========================== short test summary info ============================
FAILED kubernetes/e2e_test/test_client.py::TestClient::test_configmap_apis - ...
FAILED kubernetes/e2e_test/test_watch.py::TestClient::test_watch_configmaps
============ 2 failed, 246 passed, 2 warnings in 165.50s (0:02:45) =============

The failing tests are in the python repo. The one in this repo (python-base) has passed.

kubernetes/base/dynamic/test_client.py::TestDynamicClient::test_configmap_apis PASSED

/hold cancel