os.Rename cross-filesystem issue alternative solution

https://github.com/kelseyhightower/confd/pull/69 apparently has issues due to failing builds, so I took a look at it and would like to propose an alternative solution.

Since we're creating a tempfile, and the issue is renaming that tempfile across file systems,  why not just create the tempfile on the same filesystem as the destination file?

This seems like a much simpler change, since the tempfile is being cleaned up after it is no longer used.