Fix a bug in MD5 check triggered in wheezy

If you try snf-image-update-helper in wheezy you'll get a
"no properly formatted MD5 checksum lines found" bug, because of a
sed command that will trim 2 spaces into 1.

snf-image-host/snf-image-update-helper.in

@@ -145,7 +145,7 @@ if [ "x$NO_CHECKSUM" != "xyes" ]; then
     # curl will not attempt to decode %-sequences which may lead to unexpected
     # filenames. It's better if we just replace the filename inside the md5sum
     # file with the one we know.
-    sed -i 's/ .\+$/'" $IMAGE"'/' "$IMAGE_DIR/$IMAGE.md5sum"
+    sed -i 's/  .\+$/'"  $IMAGE"'/' "$IMAGE_DIR/$IMAGE.md5sum"
 
     $MD5SUM -c "$IMAGE_DIR/$IMAGE.md5sum"
 fi