Makes image caching more resilient

Avoids failing on the first attempt to download the image to cache as
mirrors hosting them can randomly go down, usually with a connection
refused.

Change-Id: I9de9f33c2cc16596d04b35c4eb92621e6a2c7511

diskimage_builder/elements/cache-url/bin/cache-url

@@ -61,7 +61,7 @@ done
 url=$1
 dest=$2
 time_cond=
-curl_opts=""
+curl_opts="--retry 3 --retry-delay 30 "
 
 if [ -z $url -o -z $dest ] ; then
     show_options 1
@@ -77,7 +77,7 @@ else
 fi
 
 if [ "$FORCE_REVALIDATE" = "1" ]; then
-    curl_opts="-H 'Pragma: no-cache, must-revalidate' -H 'Cache-Control: no-cache, must-revalidate'"
+    curl_opts+="-H 'Pragma: no-cache, must-revalidate' -H 'Cache-Control: no-cache, must-revalidate'"
     success="Downloaded and cached $url, having forced upstream caches to revalidate"
 elif [ -f $dest -a -s $dest ] ; then
     time_cond="-z $dest"