Remove assumptions about temp file locations

Many configuration files live in a parent directory that is only writeable by root or another user.  Despite this, current code assumes the parent directory is writable by the consul-template process.

In addition, the assumption is made that the temporary file is created on the same filesystem.

We can still retain an atomic copy+delete using the existing `copyFile(...)` and deferred `os.Remove(...)`.

This patch allows for more granular security and more variation in the host filesystem.  In my case, I run consul-template as a dedicated user, so that I can explicitly give it permission to writeable locations.

runner.go

@@ -813,7 +813,7 @@ func atomicWrite(path string, contents []byte, perms os.FileMode, backup bool) e
 		}
 	}
 
-	f, err := ioutil.TempFile(parent, "")
+	f, err := ioutil.TempFile(os.TempDir(), "")
 	if err != nil {
 		return err
 	}
@@ -845,7 +845,7 @@ func atomicWrite(path string, contents []byte, perms os.FileMode, backup bool) e
 		}
 	}
 
-	if err := os.Rename(f.Name(), path); err != nil {
+	if err := copyFile(f.Name(), path); err != nil {
 		return err
 	}