adds a quiet flag

[brendanjryan]

Adds a --quiet flag for suppressing "non terminal" i.e. error or result messages from the kubeval logs.

This will help users who are running kubeval in CI or other automated systems capture only the results they care about.

Examples:

Normal output

⇒  ./bin/kubeval -o json --ignore-missing-schemas fixtures/valid.yaml        
Warning: Set to ignore missing schemas
[
        {
                "filename": "fixtures/valid.yaml",
                "kind": "ReplicationController",
                "status": "valid",
                "errors": []
        }
]

Flag enabled

⇒  ./bin/kubeval -o json --ignore-missing-schemas --quiet fixtures/valid.yaml
[
        {
                "filename": "fixtures/valid.yaml",
                "kind": "ReplicationController",
                "status": "valid",
                "errors": []
        }
]

[brendanjryan]

cc @garethr

[garethr]

Thanks, catching up after some holiday.