Fix a potential file leak

Previously, if a write or sync error occurred, we would not have called
Close(). This commit refactors ReplaceFile() so that we are sure to call
Close(), and also attempts to delete the temporary file if errors occur.

pkg/util/filesystem/defaultfs.go

@@ -62,6 +62,11 @@ func (DefaultFs) RemoveAll(path string) error {
 	return os.RemoveAll(path)
 }
 
+// Remove via os.RemoveAll
+func (DefaultFs) Remove(name string) error {
+	return os.Remove(name)
+}
+
 // ReadFile via ioutil.ReadFile
 func (DefaultFs) ReadFile(filename string) ([]byte, error) {
 	return ioutil.ReadFile(filename)