Dont add to ssh-agent unless specified

I would suggest implementing a parameter that allows stopping bless from adding the Identity file to `ssh-agent`.
Namely this lines: https://github.com/lyft/python-blessclient/blob/master/blessclient/client.py#L594-L597

Given a config like:
```
Match exec "env | grep -q BLESS_COMPLETE || /Users/stype/blessclient/blessclient.run --gui --host '%h'"
IdentityFile ~/.ssh/blessid
```
this is not required and could interfere with other credentials.

I think this is required when doing a wrapper around the `ssh` command but in that case its easy to toggle on in the script.

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

Start in blessclient/client.py at lines 594-597 and trace how the client options are passed to the SSH-agent handling. Add a parameter that disables adding the Identity file while preserving the existing behavior by default; done means the wrapper use case can opt out without affecting other credentials.

Written by the indexing model from the issue text.