dockerd: enable shell-completion; add (hidden) completion subcommand

[thaJeztah]

This required a minor hack to accommodate Cobra's logic (and a TODO was added to see if we can improve that logic in upstream). Some changes also had to be made to our "Usage" template, as our custom template did not take into account subcommands and long descriptions for commands. We should review these templates, as some additional features were added in upstream Cobra that we currently may not be using.

With this patch:

instructions for the (hidden) completion subcommand:

docker completion --help

Usage:  docker completion COMMAND

Generate the autocompletion script for docker for the specified shell.
See each sub-command's help for details on how to use the generated script.

Commands:
  bash        Generate the autocompletion script for bash
  fish        Generate the autocompletion script for fish
  powershell  Generate the autocompletion script for powershell
  zsh         Generate the autocompletion script for zsh

Run 'docker completion COMMAND --help' for more information on a command.

and instructions for installing:

dockerd completion bash --help

Usage:	dockerd completion bash

Generate the autocompletion script for the bash shell.

This script depends on the 'bash-completion' package.
If it is not installed already, you can install it via your OS's package manager.

To load completions in your current shell session:

    source <(dockerd completion bash)

To load completions for every new session, execute once:

#### Linux:

    dockerd completion bash > /etc/bash_completion.d/dockerd

#### macOS:

    dockerd completion bash > $(brew --prefix)/etc/bash_completion.d/dockerd

You will need to start a new shell for this setup to take effect.

Options:
      --help              Print usage
      --no-descriptions   disable completion descriptions

- What I did

- How I did it

- How to verify it

- Description for the changelog

dockerd:  add (hidden) `completion` subcommand to generate shell-completion scripts

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

With this PR:

source <(dockerd completion bash)

dockerd --default-<TAB>
--default-address-pool   (Default address pools for node specific local networks)
--default-cgroupns-mode  (Default mode for containers cgroup namespace ("host" | "private"))
--default-gateway        (Container default gateway IPv4 address)
--default-gateway-v6     (Container default gateway IPv6 address)
--default-ipc-mode       (Default mode for containers ipc ("shareable" | "private"))
--default-network-opt    (Default network options)
--default-runtime        (Default OCI runtime for containers)
--default-shm-size       (Default shm size for containers)
--default-ulimit         (Default ulimits for containers)

After this PR, we need to improve the code to provide completion for flag values, so this is just the initial start.

[vvoland]

LGTM