Make subctl command variable

[Jaanki]

so that the help can display proper output when subctl is used as a standalone CLI or as kubectl-plugin.

Epic: submariner-io/enhancements#182

When used as kubectl plugin named subm, the usage and help message displayed would be

$ kubectl subm                                                                                              
An installer for Submariner

Usage:
  kubectl-subm [command]

Available Commands:
  benchmark           Benchmark tests
  cloud               Cloud operations
  completion          Generate the autocompletion script for the specified shell
  deploy-broker       Deploys the broker
  diagnose            Run diagnostic checks on the Submariner deployment and report any issues
  export              Exports a resource to other clusters
  gather              Gather troubleshooting information from a cluster
  help                Help about any command
  join                Connect a cluster to an existing broker
  recover-broker-info Recovers the broker-info.subm file from the installed Broker
  show                Show information about Submariner
  unexport            Stop a resource from being exported to other clusters
  uninstall           Uninstall Submariner and its components
  verify              Run verifications between two clusters
  version             Get version information on subctl

Flags:
  -h, --help   help for kubectl-subm

Use "kubectl-subm [command] --help" for more information about a command.

[submariner-bot]

🤖 Created branch: z_pr804/Jaanki/generalize_cmd_name
🚀 Full E2E won't run until the "ready-to-test" label is applied. I will add it automatically once the PR has 2 approvals, or you can add it manually.

[submariner-bot]

🤖 Closed branches: [z_pr804/Jaanki/generalize_cmd_name]