Change GOPATH from gerrit.opencord.org to github.com/opencord

Ran `go fmt` which did some line reordering after this change

Conformed Makefile to template the imagename and use correct DOCKER_*
build variables

Change-Id: I52734e805698b305cf64eb9238b08f152f68ffd4
diff --git a/core/core_server.go b/core/core_server.go
index 65b9c10..6c68706 100644
--- a/core/core_server.go
+++ b/core/core_server.go
@@ -24,15 +24,15 @@
 	"strconv"
 	"sync"
 
-	pb "gerrit.opencord.org/voltha-bbsim/api"
-	"gerrit.opencord.org/voltha-bbsim/common/logger"
-	"gerrit.opencord.org/voltha-bbsim/device"
-	flowHandler "gerrit.opencord.org/voltha-bbsim/flow"
-	openolt "gerrit.opencord.org/voltha-bbsim/protos"
 	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
 	"github.com/google/gopacket/pcap"
 	omci "github.com/opencord/omci-sim"
+	pb "github.com/opencord/voltha-bbsim/api"
+	"github.com/opencord/voltha-bbsim/common/logger"
+	"github.com/opencord/voltha-bbsim/device"
+	flowHandler "github.com/opencord/voltha-bbsim/flow"
+	openolt "github.com/opencord/voltha-bbsim/protos"
 	log "github.com/sirupsen/logrus"
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/grpc"