Update remote refs in early devstack clone

In another confusing addendum to
I41e81d6bac98875eecde2376e0865784626e11a8 (which was already a
confusing addendum to Ieb6a6e9f55bd93f63c3d0a71828c276c2d02e1b9), we
have decided that the refspec used to fetch here is not sufficient to
clear out remote branches everything when updating.

"+refs/heads/*:refs/heads/*" says to replicate everything from the
remote refs/heads into our local refs/heads, but leaves out
refs/remotes/*

The upshot of this is that I41e81d6bac98875eecde2376e0865784626e11a8
will remove the local branches (refs/heads/stable/icehouse, say) but
not remove the remote branches (refs/remotes/origin/stable/icehouse).
The devstack caching script keeps picking up these remote branches,
checking them out, and consequently trying to download old images.

*Nothing* ever removes these branches.  In the main dib cache git
update, we also have --prune, but our refspec there is even more
limited (+master:master).  This explains why these branches never seem
to die.

Note, an even better mirror would be "+refs/*:refs/*" (in fact, if you
do git clone --mirror, this is what the repo would be setup with to
fetch by default).  However, this drags in "refs/changes/*" and all
sorts of other gerrit things.  We don't really need them so we just
keep the limiting on.

Change-Id: Ia9c3ffdb2b5f72a45d629961338b415308d6dd21

nodepool/elements/cache-devstack/extra-data.d/50-early-source-repo

@@ -129,7 +129,8 @@ function get_repos_for_element(){
                 if [ -z "$DIB_OFFLINE" -o -z "$HAS_REF" ] ; then
                     echo "Updating cache of $REPOLOCATION in $CACHE_PATH with ref $REPOREF"
                     git --git-dir=$CACHE_PATH/.git fetch --update-head-ok --prune \
-                        $REPOLOCATION +refs/heads/*:refs/heads/*
+                        $REPOLOCATION +refs/heads/*:refs/heads/* \
+                        +refs/remotes/*:refs/remotes/*
                 fi
 
                 echo "Cloning from $REPONAME cache and applying ref $REPOREF"