Unintentional Identity file signing

Hi, while troubleshooting some unrelated problems I noticed this client is generated signed key certs for any identity that we use in the in the ssh command.
The culprit seems to be this:
https://github.com/lyft/python-blessclient/blob/master/blessclient/client.py#L171

called here:
https://github.com/lyft/python-blessclient/blob/master/blessclient/client.py#L448

Given an ssh config as recommended:

Match exec "env | grep -q BLESS_COMPLETE || /Users/stype/blessclient/blessclient.run --gui --host '%h'"
	IdentityFile ~/.ssh/blessid

If we are to call any other ssh command as:
ssh -i ~/.ssh/mykey user@host
and we didn't have a filter on domain_regex: blessclient will still generate and sign the mykey key.

While I believe this could be in some case desired functionality(when doing ssh wrapping instead of ssh config), I think it might be better to just let it toggle via an env var or the existing BLESS_IDENTITYFILE as in most cases than not if you specify a particular identity on the command line, you want to use exactly that to auth and signing is unnecessary.

If this is accepted I can create a PR to cleanup/implement this.

アイデンティティが処理され、署名パスが呼び出される blessclient/client.py の171行目と448行目から始めます。示されている SSH 設定と -i ~/.ssh/mykey を使用する ssh コマンドで動作を再現し、その後、BLESS_IDENTITYFILE と domain_regex が選択にどのような影響を与えるかを追跡します。選択した設定でその動作が有効になっていない限り、クライアントが明示的に選択されたアイデンティティに署名しなくなれば完了です。