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.

从 blessclient/client.py 的第 171 行和第 448 行开始，这里处理 identity 并调用 signing path。使用所示的 SSH 配置和带有 -i ~/.ssh/mykey 的 ssh 命令重现该行为，然后跟踪 BLESS_IDENTITYFILE 和 domain_regex 如何影响选择。当 client 不再对显式选择的 identity 进行签名，除非所选配置启用了该行为时，即表示完成。