Add command cluster get-peer-clusters

[zymap]

Master Issue: #2

output :

USED FOR:
    This command is used for getting the peer clusters of the specified cluster.

REQUIRED PERMISSION:
    This command requires super-user permissions.

EXAMPLES:
    #getting the <cluster-name> peer clusters
    pulsarctl clusters get-peer-clusters <cluster-name>

OUTPUT:
    #normal output
    +-------------------+
    |   PEER CLUSTERS   |
    +-------------------+
    | test_peer_cluster |
    +-------------------+

    #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 get-peer-clusters [flags]

Aliases: get-peer-clusters, gpc

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 get-peer-clusters [command] --help' for more information about a command.

[sijie]

@zymap please rebase the pull request.

[sijie]

please rebase the pull request