kubefed2: cleanup global and common subcommand options

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This issue captures the work needed to cleanup various global and shared subcommand options in kubefed2. Some examples:

• --registry-namespace parameter is not needed for kubefed2 <enable|disable> commands.
• NAME is not required for kubefed2 enable command as a filename option is also accepted.
• Code for join and unjoin each define options that are mutual e.g. --cluster-context and --host-cluster-name.
• Code defines CLUSTER_NAME required arg for all subcommands but only used for join and unjoin.