cmd: add generate and refactor structure

[hongchaodeng]

No description provided.

[fanminshi]

I think the original "Compiles code and builds artifacts" is fine.

for example:

Command's short description from etcdctl:
Short: "Puts the given key into the store",
Short: "Watches events stream on keys or prefixes",

I think the key here is to keep language usage and style consistent.

[fanminshi]

No need to change here.

[fanminshi]

For example, -> For example:

[fanminshi]

It is better to keep the code consistent.

just use:
var generateCmd = &cobra.Command{..}

[fanminshi]

Generate -> Generates

[fanminshi]

lgtm

[hongchaodeng]

tested:

$ operator-sdk
A sdk for building operator with ease

Usage:
  operator-sdk [command]

Available Commands:
  build       Compiles code and builds artifacts
  generate    Invokes specific generator
  help        Help about any command
  new         Creates a new operator application

Flags:
  -h, --help   help for operator-sdk

Use "operator-sdk [command] --help" for more information about a command.

$ operator-sdk build -h
The operator-sdk build command compiles the code, builds the executables,
and generates Kubernetes manifests.

<image> is the container image to be built, e.g. "quay.io/example/operator:v0.0.1".
This image will be automatically set in the deployment manifests.

After build completes, the image would be built locally in docker. Then it needs to
be pushed to remote registry.
For example:
	$ operator-sdk build quay.io/example/operator:v0.0.1
	$ docker push quay.io/example/operator:v0.0.1

Usage:
  operator-sdk build <image> [flags]

Flags:
  -h, --help   help for build

$ operator-sdk generate
The operator-sdk generate command invokes specific generator to generate code as needed.

Usage:
  operator-sdk generate [command]

Available Commands:
  k8s         Generates Kubernetes code for custom resource

Flags:
  -h, --help   help for generate

Use "operator-sdk generate [command] --help" for more information about a command.

$ operator-sdk new -h
The operator-sdk new command creates a new operator application and
generates a default directory layout based on the input <project-name>.

<project-name> is the project name of the new operator. (e.g app-operator)

	--api-version and --kind are required flags to generate the new operator application.

For example:
	$ mkdir $GOPATH/src/github.com/example.com/
	$ cd $GOPATH/src/github.com/example.com/
	$ operator-sdk new app-operator --api-group=app.example.com --kind=AppService
generates a skeletal app-operator application in $GOPATH/src/github.com/example.com/app-operator.

Usage:
  operator-sdk new <project-name> [required-flags] [flags]

Flags:
      --api-version string   Kubernetes apiVersion and has a format of $GROUP_NAME/$VERSION (e.g app.example.com/v1alpha1)
  -h, --help                 help for new
      --kind string          Kubernetes CustomResourceDefintion kind. (e.g AppService)