Rename flags to be more explicit

[dgtlntv]

The --name flag in the cluster add command and the --manifest-file in the manifest generatecommand could be more explicit in their expected values.

The --name flag could be renamed to --fqdn as the FQDN of the joining node is what is expected to be passed in this flag.

The --manifest-file in the manifest generatecommand could be renamed to --output/-o as that seems to be the standard amongst CLIs as the flag when an ouput path for a file needs to be provided.