Add command cluster update (streamnative/pulsar-admin-go#17)

Master issue: streamnative/pulsar-admin-go#2 

```
➜  pulsarctl git:(cluster_update) ✗ ./pulsarctl clusters update -h
USED FOR:
    This command is used for updating the cluster data of the specified cluster.

REQUIRED PERMISSION:
    This command requires super-user permissions.

EXAMPLES:
    #updating the web service url of the <cluster-name>
    pulsarctl clusters update --url http://example:8080 <cluster-name>

    #updating the tls secured web service url of the <cluster-name>
    pulsarctl clusters update --url-tls https://example:8080 <cluster-name>

    #updating the broker service url of the <cluster-name>
    pulsarctl clusters update --broker-url pulsar://example:6650 <cluster-name>

    #updating the tls secured web service url of the <cluster-name>
    pulsarctl clusters update --broker-url-tls pulsar+ssl://example:6650 <cluster-name>

    #registered as a peer-cluster of the <cluster-name> clusters
    pulsarctl clusters update -p <cluster-a>,<cluster-b> <cluster>

OUTPUT:
    #normal output
    Cluster <cluster-name> updated

    #output of doesn't specified  a cluster name
    [✖]  only one argument is allowed to be used as a name

    #Output of cluster doesn't exist
    [✖]  code: 404 reason: Cluster does not exist

Usage: pulsarctl clusters update [flags]

Aliases: update, update

ClusterData flags:
      --url string                 Pulsar cluster web service url, e.g. http://example.pulsar.io:8080
      --url-tls string             Pulsar cluster tls secured web service url, e.g. https://example.pulsar.io:8443
      --broker-url string          Pulsar cluster broker service url, e.g. pulsar://example.pulsar.io:6650
      --broker-url-tls string      Pulsar cluster tls secured broker service url, e.g. pulsar+ssl://example.pulsar.io:6651
  -p, --peer-cluster stringArray   Cluster to be registered as a peer-cluster of this cluster.

Common flags:
  -s, --admin-service-url string   The admin web service url that pulsarctl connects to. (default "http://localhost:8080")
  -C, --color string               toggle colorized logs (true,false,fabulous) (default "true")
  -h, --help                       help for this command
  -v, --verbose int                set log level, use 0 to silence, 4 for debugging (default 3)

Use 'pulsarctl clusters update [command] --help' for more information about a command.
```

pulsaradmin/pkg/pulsar/cluster.go

@@ -7,17 +7,18 @@ type Clusters interface {
 	Get(string) (ClusterData, error)
 	Create(ClusterData) error
 	Delete(string) error
+	Update(ClusterData) error
 }
 
 type clusters struct {
-	client 		*client
-	basePath 	string
+	client   *client
+	basePath string
 }
 
 func (c *client) Clusters() Clusters {
 	return &clusters{
-		client: c,
-		basePath:"/clusters",
+		client:   c,
+		basePath: "/clusters",
 	}
 }
 
@@ -45,4 +46,7 @@ func (c *clusters) Delete(name string) error {
 	return c.client.delete(endpoint, nil)
 }
 
-
+func (c *clusters) Update(cdata ClusterData) error {
+	endpoint := c.client.endpoint(c.basePath, cdata.Name)
+	return c.client.post(endpoint, &cdata, nil)
+}

pulsaradmin/pkg/pulsar/descriptions.go

@@ -31,15 +31,15 @@ func (desc *LongDescription) ToString() string {
 		PERMISSION + "\n" +
 		SPACES + desc.CommandPermission + "\n\n" +
 		EXAMPLES + "\n" +
-		desc.exampleToString() + "\n" +
+		desc.exampleToString() +
 		OUTPUT + "\n" +
 		desc.outputToString()
 }
 
 func (desc *LongDescription) exampleToString() string {
 	var result string
 	for _, v := range desc.CommandExamples {
-		result += SPACES + "#" + v.Desc + "\n" + SPACES + v.Command + "\n"
+		result += SPACES + "#" + v.Desc + "\n" + SPACES + v.Command + "\n\n"
 	}
 	return result
 }

pulsaradmin/pkg/pulsar/descriptions_test.go

@@ -15,7 +15,7 @@ func TestLongDescription_exampleToString(t *testing.T) {
 	res := desc.exampleToString()
 
 	expect := "    #command description\n" +
-		"    command\n"
+		"    command\n\n"
 
 	assert.Equal(t, expect, res)
 }