Containerd client integration

This commit:
1) Replaces the usage of containerd GRPC APIs with the containerd client for all operations related to containerd.
2) Updated containerd to v1.0alpha4+
3) Updated runc to v1.0.0

Signed-off-by: Abhinandan Prativadi <abhi@docker.com>

vendor/github.com/opencontainers/image-spec/specs-go/v1/config.go

@@ -37,7 +37,7 @@ type ImageConfig struct {
 	// Cmd defines the default arguments to the entrypoint of the container.
 	Cmd []string `json:"Cmd,omitempty"`
 
-	// Volumes is a set of directories which should be created as data volumes in a container running this image.
+	// Volumes is a set of directories describing where the process is likely write data specific to a container instance.
 	Volumes map[string]struct{} `json:"Volumes,omitempty"`
 
 	// WorkingDir sets the current working directory of the entrypoint process in the container.

vendor/github.com/opencontainers/image-spec/specs-go/version.go

@@ -25,7 +25,7 @@ const (
 	VersionPatch = 0
 
 	// VersionDev indicates development branch. Releases will be empty string.
-	VersionDev = "-rc6-dev"
+	VersionDev = ""
 )
 
 // Version is the specification version that the package types support.