Introduce new command to generate kubeconfig

[gianlucam76]

sveltosctl generate kubeconfig --help
Usage:
  sveltosctl generate kubeconfig [options] [--namespace=<name>] [--serviceaccount=<name>] [--create] [--verbose]

     --namespace=<name>      The namespace of the ServiceAccount. If not specified, projectsveltos namespace will be used.
     --serviceaccount=<name> The name of the ServiceAccount. If not specified, projectsveltos will be used.
     --create                If a ServiceAccount with enough permissions is already present, do not set this flag.
                             Sveltos will generate a Kubeconfig associated to that ServiceAccount.
                             If a ServiceAccount with cluster admin permissions needs to be created, use this option.
                             When this option is set, this command will create necessary resources:
                             1. namespace if not existing already
                             2. serviceAccount if not existing already
                             3. ClusterRole with cluster admin permission
                             4. ClusterRoleBinding granting the serviceAccount cluster admin permissions
                             5. TokenRequest for the ServiceAccount

Options:
  -h --help                  Show this screen.
     --verbose               Verbose mode. Print each step.

Description:
  The generate kubeconfig command will generate a Kubeconfig that can later on be used to register the cluster.

This PR removes the sveltosctl register mgmt-cluster

To register the management cluster with Sveltos, simply

1. run sveltosctl generate kubeconfig ... against the management cluster
2. run sveltosctl register cluster ... using the Kubeconfig generated in the previous step