Add command cluster delete

[zymap]

Master issue: #2

Output:

➜  pulsarctl git:(clusters_delete) ./pulsarctl clusters delete -h
USED FOR:
    This command is used for deleting an existing cluster.

REQUIRED PERMISSION:
    This command requires super-user permissions.

EXAMPLES:
    #deleting the cluster named <cluster-name>
    pulsarctl clusters delete <cluster-name>

OUTPUT:
    #normal output
    Cluster <cluster-name> delete successfully.

    #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 delete [flags]

Aliases: delete, delete

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 delete [command] --help' for more information about a command.

[wolfstudy]

Left a few comments

[sijie]

two problems here:

• whenever you are changing the logic, please write the change in the description of the pull request. otherwise no one knows why you are making this change.

• this command requires super user permissions: https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/ClustersBase.java#L118