Remove output formats from the Yaml file, put in CLI

This removes outputs from yaml, instead you can do
```
moby build -output tar -output qcow2 file.yaml
```
or alternative syntax
```
moby build -output tar,qcow2 file.yaml
```

In future we may change this to be available in a `moby package`
step, but lets try this for now.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

test/test.yml

@@ -56,5 +56,3 @@ trust:
     - linuxkit/kernel
     - linuxkit/binfmt
     - linuxkit/rngd
-outputs:
-  - format: tar