R1:cb1d90465447

Make sure writes go to the right cluster

Summary:
Two little issues

1. there was an extra call to getHostForWrite,
2. The engine instance was shared between multiple service definitions so it
was overwriting the list of writable hosts from one service with hosts from another.

Test Plan:
tested in wikimedia production with multiple services defined like this:

```language=json
[
{
"hosts": [
{
"host": "search.svc.codfw.wmnet",
"protocol": "https"…