Addind the possibility to list existing relation with the kubectl volsync pluging

[fabricekrebs]

When managing relationship with volsync, relation name needs to be passed in parameters.

It isn't very convenient to know which relation is set as of today. The kubectl volsync plugin allows to create, delete and manage relation, but doesn't allow to list existing one. It should be able to list the existing relation.

Existing commands

Usage:
  kubectl volsync replication [command]

Available Commands:
  create          Create a new replication relationship
  delete          Delete an existing replication relationship
  schedule        Set replication schedule for the relationship
  set-destination Set the destination of the replication
  set-source      Set the source of the replication
  sync            Run a single synchronization

Ideal commands should be similar to this

Usage:
  kubectl volsync replication [command]

Available Commands:
  create          Create a new replication relationship
  delete          Delete an existing replication relationship
  schedule        Set replication schedule for the relationship
  set-destination Set the destination of the replication
  set-source      Set the source of the replication
  sync            Run a single synchronization
  list            List existing relation