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/mediator.go b/core/mediator.go
index b1d33b6..66840bb 100644
--- a/core/mediator.go
+++ b/core/mediator.go
@@ -25,8 +25,8 @@
 	"strings"
 	"sync"
 
-	"gerrit.opencord.org/voltha-bbsim/common/logger"
-	"gerrit.opencord.org/voltha-bbsim/device"
+	"github.com/opencord/voltha-bbsim/common/logger"
+	"github.com/opencord/voltha-bbsim/device"
 	log "github.com/sirupsen/logrus"
 )