Kubernetes/OpenShift Provider: Don't Require Bash in Container Images

Currently the Kubernetes and OpenShift providers set the entrypoint
of their build node pods to `/bin/bash`, which then requires `bash`
to be available in the respective container image. This might not
always be the case (e.g. with Alpine based images).
This change makes sure the entrypoint is set to `/bin/sh`, which we
can more reliably assume to be available in the container image.

Change-Id: I799ea95b715e50d9c22e66cc80579cf119db8f38

nodepool/driver/kubernetes/provider.py

@@ -277,7 +277,7 @@ class KubernetesProvider(Provider):
                     'name': label.name,
                     'image': label.image,
                     'imagePullPolicy': label.image_pull,
-                    'command': ["/bin/bash", "-c", "--"],
+                    'command': ["/bin/sh", "-c"],
                     'args': ["while true; do sleep 30; done;"],
                     'workingDir': '/tmp'
                 }]

nodepool/driver/openshift/provider.py

@@ -205,7 +205,7 @@ class OpenshiftProvider(Provider):
             'name': label.name,
             'image': label.image,
             'imagePullPolicy': label.image_pull,
-            'command': ["/bin/bash", "-c", "--"],
+            'command': ["/bin/sh", "-c"],
             'args': ["while true; do sleep 30; done;"],
             'workingDir': '/tmp',
         }