fix lint errors

Some lint errors were introduced with the preflight-check merge.

golint -set_exit_status ./...
cmd/preflight_check.go:223:9: if block ends with a return statement, so drop this else and outdent its block
cmd/preflight_check.go:256:9: if block ends with a return statement, so drop this else and outdent its block
util/helm/client.go:14:6: exported type GetReleaseFunc should have comment or be unexported
util/helm/client.go:15:6: exported type GetListFunc should have comment or be unexported
util/helm/client.go:16:6: exported type GetValuesFunc should have comment or be unexported
util/helm/client.go:18:6: exported type HelmClient should have comment or be unexported
util/helm/client.go:18:6: type name will be used as helm.HelmClient by other packages, and that stutters; consider calling this Client
util/helm/client.go:24:1: comment on exported function NewClient should be of the form "NewClient ..."
util/k8s/job.go:12:6: exported type JobDesc should have comment or be unexported
util/k8s/job.go:22:1: exported function CreateJob should have comment or be unexported
util/k8s/namespace.go:11:1: exported function CreateNamespace should have comment or be unexported
util/k8s/namespace.go:21:1: exported function DeleteNamespace should have comment or be unexported
util/k8s/secret.go:30:2: struct field HttpHeaders should be HTTPHeaders
util/k8s/secret.go:33:1: exported function CreateRegistrySecret should have comment or be unexported
util/k8s/secret.go:68:1: exported function DeleteRegistrySecret should have comment or be unexported
Found 15 lint suggestions; failing.