SEBA-806:Bring up ONUs one by one

For simulating onu discovery in a frequency, a time gap inserted between the OnuDiscovery messages generated by BBSIM.
For this, the configurable IndicationInterval parameter is used.

Change-Id: I49e2afb57e8321550898cbe89ee8b17ef580dc44
diff --git a/core/core_server.go b/core/core_server.go
index 6c68706..6899d86 100644
--- a/core/core_server.go
+++ b/core/core_server.go
@@ -23,6 +23,7 @@
 	"reflect"
 	"strconv"
 	"sync"
+	"time"
 
 	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
@@ -65,7 +66,7 @@
 	mgmtGrpcPort    uint32
 	mgmtRestPort    uint32
 	Vethnames       []string
-	IndInterval     int
+	IndInterval     time.Duration
 	Processes       []string
 	EnableServer    *openolt.Openolt_EnableIndicationServer
 	CtagMap         map[string]uint32
@@ -363,6 +364,9 @@
 	for intfid := range Onumap {
 		for _, onu := range Onumap[intfid] {
 			s.activateOnu(onu)
+			if s.IndInterval > 0 {
+				time.Sleep(s.IndInterval)
+			}
 		}
 	}
 }