feat: add flag for developing dy admin apply

[wafuwafu13]

*Issue #155 (comment)

Description of changes:

During development, to eliminate the impact on users, ensure that apply only works when --dev is added.

# just return message
$ dy admin apply
not yet implemented

# develop feature
$ dy admin apply --dev
(something happen)

Now, dy admin apply --dev causes panic.

~/Desktop/dynein
$ ./target/release/dy admin apply
not yet implemented

~/Desktop/dynein
$ ./target/release/dy admin apply --dev
thread 'main' panicked at 'not yet implemented', src/main.rs:89:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[StoneDot]

Thank you for trying to build new feature for dynein.
I understand that you are trying to create the feature in separate multiple pull requests. I think It is a reasonable tactic.

As my quick search, I found that hidden option of structopt may work for this kind of phased development. I hope this information helps to implement more user-friendly interface even in developing phase. TeXitoi/structopt#466

[wafuwafu13]

Do not display apply subcommand in help is good idea but it is written in the README.
So I want to keep --test-apply flag so that even if the user executes it, it will not affect the user.

[StoneDot]

Could you check my comments? I think you need to sort out what you want to check on your test code. I'm terrible sorry, but I was not able to get your intention from the code.

[StoneDot]

Could you check a new comment?

[StoneDot]

Thank you for the fix. LGTM