Use consul/lib's RandomStagger

Removes four redundant copies of the method in the process.

command/agent/util.go

@@ -2,16 +2,9 @@ package agent
 
 import (
 	"fmt"
-	"math/rand"
 	"net"
-	"time"
 )
 
-// Returns a random stagger interval between 0 and the duration
-func randomStagger(intv time.Duration) time.Duration {
-	return time.Duration(uint64(rand.Int63()) % uint64(intv))
-}
-
 // IpOfDevice returns a routable ip addr of a device
 func ipOfDevice(name string) (net.IP, error) {
 	intf, err := net.InterfaceByName(name)