Fix download upper constraints

We use the url from tox.ini to download the upper constraints and
recently that has changed to a redirect and curl doesn't
correctly follows redirects by default but stops at the first
page.
Adding -L option to curl command makes curl follows the redirect
until the final location is found and should allow the correct
file to be downloaded.

Change-Id: I25c724ffe189d414643c910be0ef61c550f802b7
(cherry picked from commit a88e61959b)

imagebuild/common/generate_upper_constraints.sh

@@ -31,7 +31,7 @@ download() {
         return 1
     else
         log "Downloading from '${url}'"
-        curl ${url} -o "${destination}"
+        curl -L ${url} -o "${destination}"
     fi
     return 0
 }