Annotations: Provide container metadata for VM based runtimes

For hypervisor-based container runtimes (like Kata Containers, Clear Containers
or runv) a pod will be created in a VM and then create containers within the VM.

When a runtime is requested for container commands like create and start, both
the instal "pause" container and next containers need to be added to the pod
namespace (same VM).

A runtime does not know if it needs to create/start a VM or if it needs to add a
container to an already running VM pod.

This patch adds a way to provide this information through container annotations.
When starting a container or a sandbox, 2 annotations are added:

- type (Container or Sandbox)
- sandbox name

This allow to a VM based runtime to decide if they need to create a pod VM or
container within the VM pod.

Signed-off-by: Jose Carlos Venegas Munoz <jose.carlos.venegas.munoz@intel.com>

pkg/annotations/annotations.go

@@ -0,0 +1,32 @@
+/*
+Copyright 2018 The Containerd 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.
+*/
+
+package annotations
+
+// ContainerType values
+const (
+	// ContainerTypeSandbox represents a pod sandbox container
+	ContainerTypeSandbox = "sandbox"
+
+	// ContainerTypeContainer represents a container running within a pod
+	ContainerTypeContainer = "container"
+
+	// ContainerType is the container type (sandbox or container) annotation
+	ContainerType = "io.kubernetes.cri.container-type"
+
+	// SandboxID is the sandbox ID annotation
+	SandboxID = "io.kubernetes.cri.sandbox-id"
+)