Add an e2e test for exec liveness probes. Fix the docker exec integration.

examples/liveness/exec-liveness.yaml

@@ -0,0 +1,22 @@
+apiVersion: v1beta1
+desiredState:
+  manifest:
+    containers:
+      - image: busybox
+        name: liveness
+        livenessProbe:
+          exec:
+            command:
+              - "cat"
+              - "/tmp/health"
+          initialDelaySeconds: 15
+        command:
+          - "/bin/sh"
+          - "-c"
+          - "echo ok > /tmp/health; sleep 10; echo fail > /tmp/health; sleep 600"
+    id: liveness-exec
+    version: v1beta1
+id: liveness-exec
+kind: Pod
+labels:
+  test: liveness