Fix ssh_auth pulling keyfile from correct env

When the ssh_auth state is reading the key from a file on the Salt fileserver, the proper environment isn't passed in. If the key file is in a non-'base' environment it fails.

Looks like this got broken in fbfd926 and affects the 2014.1.0RC3 branch. It was not broken in v0.17.4. Trying to get this in before the release, since it breaks my state files!

Not sure if pull requests for bug fixes in 2014.1 go against develop or against 2014.1 branch so either this pull request or https://github.com/saltstack/salt/pull/10197 (which merges into 2014.1) should be closed.