exec credential provider: first integration test (static auth interaction)

I also added some simple happy/sad path tests so we get some coverage at the
integration level.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

test/integration/client/testdata/exec-plugin.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Copyright 2018 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+echo "$EXEC_PLUGIN_OUTPUT"