NATS streaming cluster id missing override environment variable

[ewilde]

Description

There is no way to change the name of the nats cluster id used for asynchronous invocation. Currently it expects the cluster id to be faas-cluster.

Note that the queue-worker does allow the cluster id to be overridden https://github.com/openfaas/nats-queue-worker/blob/dea1c90b8cc66dc73597b7531a4fd29a32b5f88c/readconfig.go#L42

Proposal

Add environment override to gateway default it to faas-cluster