Merge plugin into weaver

This simplifies the following:

- plugin-v2 does not need to start "plugin" as a separate process, thus
  Docker is able to properly restart plugin-v2 in a case of any child
  process failing.
- No need for weaveplugin container.

plugin is enabled by default when running "weave launch". when starting
only the router, you should pass "--plugin" parameter to enable it.

The env var WEAVE_NO_PLUGIN is no more.

api/weaveapi.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 )
 
 const (
@@ -76,6 +77,20 @@ func (client *Client) Connect(remote string) error {
 	return err
 }
 
+// IsReady returns true if the API server is up and running
+func (client *Client) IsReady() bool {
+	_, err := client.httpVerb("GET", "/status", nil)
+
+	return err == nil
+}
+
+// WaitAPIServer waits until the API server is ready to serve.
+func (client *Client) WaitAPIServer(n int) {
+	for i := 0; i < n && !client.IsReady(); i++ {
+		time.Sleep(2 * time.Second)
+	}
+}
+
 type Logger interface {
 	Infof(string, ...interface{})
 	Debugf(string, ...interface{})