cmd/deploy: update usage string

General MR

Summary

Update the base bbctl deploy usage string to be a little clear that this is a parent command. Reworded the error messages to be more similar to those from kubectl and helm. Notably, it now lists valid options and prints the help string by default.

Relevant logs/screenshots

Before

bbctl deploy
Please provide a subcommand for deploy (see help)

After

bbctl deploy
error: must specify one of: bigbang, flux

Deploy bigbang components and prerequisites.

 Note: Before deploying bigbang, you must first deploy flux into the cluster. See "bigbang deploy flux --help" for more information.

Usage:
  bbctl deploy [flags]
  bbctl deploy [command]

Available Commands:
  bigbang     deploy big bang components to your cluster
  flux        Deploy flux to your kubernetes cluster

Flags:
  -h, --help   help for deploy

Linked Issue

Part of #161 (closed)

Upgrade Notices

N/A