justinsb: Set SilenceUsage on root so we don't print usage

JobBuild
12078
07-30 14:25
pull-kops-bazel-build 1421114684949401600
pull-kops-bazel-test 1421114684987150336
pull-kops-e2e-cni-canal 1444292274669228032
pull-kops-e2e-cni-flannel 1444292274698588160
pull-kops-e2e-kubernetes-aws 1421114684899069952
pull-kops-verify-bazel 1421114685016510464
pull-kops-verify-boilerplate 1421114685138145280
pull-kops-verify-generated 1421114685041676288
pull-kops-verify-gofmt 1421114685146533888
pull-kops-verify-gomod 1421114685087813632
pull-kops-verify-govet 1421114685196865536
pull-kops-verify-staticcheck 1421114685217837056