use ssh agent when environment indicates it and no key was configured

Using Rex only with an ssh agent but no local keys always gives this warning for every host:

Warning: Identity file $HOME/.ssh/id_rsa not accessible: No such file or directory.

This commit just prevents default file locations when SSH_AUTH_SOCK is set,
maybe a better fix would be a file check. But then again the full list of possible
keys would have to be checked and I wonder if an empty default would not simply
do the trick instead.
Any thoughts on that?

Cheers,
Simon