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/go.mod b/go.mod
index 40928b7..ad2890c 100644
--- a/go.mod
+++ b/go.mod
@@ -1,4 +1,4 @@
-module gerrit.opencord.org/voltha-bbsim
+module github.com/opencord/voltha-bbsim
 
 go 1.12