schedulers/local_scheduler: silence docker version output (#290)

Summary:
88a3f41 accidentally added some extra log spam from `docker version` this hides it.

Pull Request resolved: #290

Test Plan:
```
tristanr@tristanr-arch2 ~/D/torchx (dockerquiet)> torchx run  utils.echo
0.1.0rc2: Pulling from pytorch/torchx
Digest: sha256:f97df7bd3d2137b3dcb6b85e14f4eef9c1c7cdd826d12508cc7cafb08bb2f704
Status: Image is up to date for ghcr.io/pytorch/torchx:0.1.0rc2
ghcr.io/pytorch/torchx:0.1.0rc2
local_docker://torchx/echo_484dcf48
torchx 2021-10-20 14:23:25 INFO     Waiting for the app to finish...
WARNING: IPv4 forwarding is disabled. Networking will not work.
hello world
torchx 2021-10-20 14:23:27 INFO     Job finished: SUCCEEDED
```

Reviewed By: aivanou

Differential Revision: D31809970

Pulled By: d4l3k

fbshipit-source-id: cac5076e8cf9f5ca691fdefe1a31c07fe17b1e81

Author: d4l3k

torchx/schedulers/local_scheduler.py

@@ -230,7 +230,14 @@ def __init__(self, cfg: RunConfig) -> None:
     @staticmethod
     def has_docker() -> bool:
         try:
-            return subprocess.run(["docker", "version"]).returncode == 0
+            return (
+                subprocess.run(
+                    ["docker", "version"],
+                    stdout=subprocess.DEVNULL,
+                    stderr=subprocess.DEVNULL,
+                ).returncode
+                == 0
+            )
         except FileNotFoundError:
             return False