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.

Start in blessclient/client.py at lines 171 and 448, where the identity is handled and the signing path is called. Reproduce the behavior with the shown SSH config and an ssh command using -i ~/.ssh/mykey, then trace how BLESS_IDENTITYFILE and domain_regex affect selection. Done means the client no longer signs an explicitly selected identity unless the chosen configuration enables that behavior.