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/dhcp.go b/core/dhcp.go
index ae890ba..b885ec5 100644
--- a/core/dhcp.go
+++ b/core/dhcp.go
@@ -26,9 +26,9 @@
 	"reflect"
 	"sync"
 
-	"gerrit.opencord.org/voltha-bbsim/common/logger"
 	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
+	"github.com/opencord/voltha-bbsim/common/logger"
 )
 
 // Constants for DHCP states