Add a 5 second delay between cache update retries

A recent change added retry behavior to DIBs git repo caching tooling.
We've since found that all of these retries can occur within just a
couple of seconds. This may not be long enough for load balancers to
notice problems with backend services and fail over so all five retries
fail in quick succession. Add a 5 second sleep between attempts to
mitigate this problem.

Change-Id: I76a10203fa9feb4a2d7d2b90231f18ec0e94a307
Signed-off-by: Clark Boylan <clark.boylan@gmail.com>

diskimage_builder/elements/source-repositories/extra-data.d/98-source-repositories

@@ -132,6 +132,9 @@ function get_repos_for_element(){
                             echo "Attempt $attempt failed. Trying again..."
                             rm -rf $CACHE_PATH.tmp
                             attempt=$((attempt + 1))
+                            if [ $attempt -le $max_attempts ] ; then
+                                sleep 5
+                            fi
                         fi
                         set -e
                     done
@@ -169,6 +172,9 @@ function get_repos_for_element(){
                             else
                                 echo "Attempt $attempt failed. Trying again..."
                                 attempt=$((attempt + 1))
+                                if [ $attempt -le $max_attempts ] ; then
+                                    sleep 5
+                                fi
                             fi
                         set -e
                         done