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 https://github.com/saltstack/salt/commit/fbfd926fdd0c7686b6be4da1dbfe4e9e18bbea60 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!