Move plugin registration to separate package

Only compile in plugin support on non-windows 1.8.
Re-enable go 1.7.x tests.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

plugin/plugin.go

@@ -0,0 +1,73 @@
+package plugin
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/docker/containerd"
+	"github.com/docker/containerd/content"
+
+	"golang.org/x/net/context"
+	"google.golang.org/grpc"
+)
+
+type PluginType int
+
+const (
+	RuntimePlugin PluginType = iota + 1
+	GRPCPlugin
+)
+
+type Registration struct {
+	Type   PluginType
+	Config interface{}
+	Init   func(*InitContext) (interface{}, error)
+}
+
+type InitContext struct {
+	Root     string
+	State    string
+	Runtimes map[string]containerd.Runtime
+	Store    *content.Store
+	Config   interface{}
+	Context  context.Context
+}
+
+type Service interface {
+	Register(*grpc.Server) error
+}
+
+var register = struct {
+	sync.Mutex
+	r map[string]*Registration
+}{
+	r: make(map[string]*Registration),
+}
+
+// Load loads all plugins at the provided path into containerd
+func Load(path string) (err error) {
+	defer func() {
+		if v := recover(); v != nil {
+			rerr, ok := v.(error)
+			if !ok {
+				rerr = fmt.Errorf("%s", v)
+			}
+			err = rerr
+		}
+	}()
+	return loadPlugins(path)
+}
+
+func Register(name string, r *Registration) error {
+	register.Lock()
+	defer register.Unlock()
+	if _, ok := register.r[name]; ok {
+		return fmt.Errorf("plugin already registered as %q", name)
+	}
+	register.r[name] = r
+	return nil
+}
+
+func Registrations() map[string]*Registration {
+	return register.r
+}