Make an error for an invalid orchestrator

[mat007]

- What I did

I made an invalid specified orchestrator yield an error instead of being silently ignored.

- How I did it

I added checks and error handling.

- How to verify it

Before if

$ cat ~/.docker/config.json | grep orchestrator
  "orchestrator": "kubernetes",

then (notice the missing r in swarm)

$ docker --orchestrator=swam stack ls
NAME                SERVICES            ORCHESTRATOR        NAMESPACE
dtcccccc            5                   Kubernetes          default

the orchestrator flag value being invalid it falls back to the one defined in ~/.docker/config.json.

Now it correctly fails with

$ docker --orchestrator=swam stack ls
specified orchestrator "swam" is invalid, please use either kubernetes or swarm

- Description for the changelog

An invalid orchestrator now generates an error instead of being silently ignored.

- A picture of a cute animal (not mandatory but encouraged)

[cpuguy83]

LGTM

[vdemeester]

LGTM 🏑