Add cluster command update

[zymap]

Master issue: #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.

[zymap]

This has done. #17