Introduce completion

[d-kuro]

refs: #354

📝

Completion is currently not available via kubectl plugin.

ref: kubernetes/kubernetes#105867

Introduce completion for kubectl-moco.
The completion subcommand is now available by default in cobra.

$ kubectl-moco -h
the utility command for MOCO.

Usage:
  kubectl-moco [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
<snip>

Subcommand completion is currently available.

$ kubectl-moco [tab]
-- completions --
completion  -- Generate the autocompletion script for the specified shell
credential  -- Fetch the credential of a specified user
help        -- Help about any command
mysql       -- Run mysql command in a specified MySQL instance
switchover  -- Switch the primary instance

This PR enables cluster name completion and flag completion.

e.g.

$ kubectl-moco mysql -n foo [tab]
-- completions --
test

$ kubectl-moco credential --format [tab]
-- completions --
mycnf  plain

[masa213f]

LGTM