Erased `Info` struct before unmouting volumes in Docker volume isolator.

Currently when `DockerVolumeIsolatorProcess::cleanup()` is called, we will
unmount the volume first, and if the unmount operation fails we will NOT
erase the container's `Info` struct from `infos`. This is problematic
because the remaining `Info` in `infos` will cause the reference count of
the volume is greater than 0, but actually the volume is not being used by
any containers. That means we may never get a chance to unmount this volume
on this agent, furthermore if it is an EBS volume, it cannot be used by any
tasks launched on any other agents since a EBS volume can only be attached
to one node at a time. The only workaround would manually unmount the volume.

So in this patch `DockerVolumeIsolatorProcess::cleanup()` is updated to erase
container's `Info` struct before unmounting volumes.

diff --git a/src/slave/containerizer/mesos/isolators/docker/volume/isolator.cpp b/src/slave/containerizer/mesos/isolators/docker/volume/isolator.cpp
index 7be9396..5be7572 100644
--- a/src/slave/containerizer/mesos/isolators/docker/volume/isolator.cpp
+++ b/src/slave/containerizer/mesos/isolators/docker/volume/isolator.cpp
@@ -573,6 +573,13 @@
+  // Erase the `Info` struct of this container before unmounting the volumes.
+  // This is to ensure the reference count of the volume will not be wrongly
+  // increased if unmounting volumes fail, otherwise next time when another
+  // container using the same volume is destroyed, we would NOT unmount the
+  // volume since its reference count would be larger than 1.
+  infos.erase(containerId);
   return await(futures)
@@ -586,8 +593,6 @@
     const ContainerID& containerId,
     const list<Future<Nothing>>& futures)
-  CHECK(infos.contains(containerId));
   vector<string> messages;
   foreach (const Future<Nothing>& future, futures) {
     if (!future.isReady()) {
@@ -612,9 +617,6 @@
   LOG(INFO) << "Removed the checkpoint directory at '" << containerDir
             << "' for container " << containerId;
-  // Remove all this container's docker volume information from infos.
-  infos.erase(containerId);
   return Nothing();