Enforce a single search registry

Enforce "docker.io" to be the only search registry.  Short-name
resolution for remote clients is not fully supported since there is no
means to prompt.  Enforcing a single registry  works around the problem
since prompting only fires with more than one search registry.

Without this patch docker/podman client not able to pull short-name images when using remote socket.
```
$ ssh -nNT -L $(pwd)/docker.sock:/run/podman/podman.sock -i /Users/prkumar/.ssh/podman-machine-default -p 64940 root@localhost
==== docker client ====
$ docker -H unix://$(pwd)/docker.sock pull httpd:latest
Error response from daemon: failed to resolve image name: short-name resolution enforced but cannot prompt without a TTY
=== podman client ===
$ podman -c unix://$(pwd)/docker.sock pull httpd:latest
Error: short-name resolution enforced but cannot prompt without a TTY
```

- containers/podman@6f36a47
- containers/podman#11489

fcos-config.yaml

@@ -182,3 +182,9 @@ storage:
       contents:
         inline: |
           podman.crc.testing
+    - path: /etc/containers/registries.conf.d/999-podman-machine.conf
+      mode: 0644
+      contents:
+        inline: |
+          unqualified-search-registries=["docker.io"]
+