Prune old branches when updating cache

dib builds are failing in nodepool at the moment trying to download a
really old Fedora image.  What happens is that the devstack cache
script [1] goes through all the branches of the devstack checkout and
uses the inbuilt image-list tool [2] to get its list of images to
pre-download.  One of the old Fedora images required on the retired
branches has now gone, so the build halts with a 404 trying to get it.

Thus make sure we do a --prune when we fetch changes so that old
branches are removed from the cache.

[1] https://git.openstack.org/cgit/openstack-infra/project-config/tree/nodepool/elements/cache-devstack/extra-data.d/55-cache-devstack-repos
[2] https://git.openstack.org/cgit/openstack-dev/devstack/tree/tools/image_list.sh

Change-Id: Ieb6a6e9f55bd93f63c3d0a71828c276c2d02e1b9

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

@@ -125,7 +125,7 @@ function get_repos_for_element(){
                     echo "Updating cache of $REPOLOCATION in $CACHE_PATH with ref $REPOREF"
                     # Copy named refs (which might be outside the usual heads
                     # pattern) - e.g. gerrit
-                    if [ "$REPOREF" == "*" ] || ! git --git-dir=$CACHE_PATH/.git fetch --update-head-ok $REPOLOCATION \
+                    if [ "$REPOREF" == "*" ] || ! git --git-dir=$CACHE_PATH/.git fetch --prune --update-head-ok $REPOLOCATION \
                             +${REPOREF}:${REPOREF} ; then
                         # Copy all heads from the remote repository - this permits
                         # using a SHA1 object reference so long as the object