Fetch external URL from EC2 only on fresh installations

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

changelogs/unreleased/5018-external-url-ec2.yml

+---
+title: Fetch external URL from EC2 only on fresh installations
+merge_request: 3878
+author:
+type: fixed

config/templates/package-scripts/external_url.sh

@@ -38,9 +38,14 @@ set_protocol()
 }
 if [ -z "${EXTERNAL_URL}" ]; then
-  check_if_ec2
-  if [ $? -eq 0 ]  ; then
-    get_details_from_ec2
+  # Grab address from EC2 only if this is first installation. For upgrades,
+  # EXTERNAL_URL env variable is the only thing that may define external URL
+  # other than what is in gitlab.rb
+  if ! [ -e /etc/gitlab/gitlab.rb ] ; then
+    check_if_ec2
+    if [ $? -eq 0 ]  ; then
+      get_details_from_ec2
+    fi
   fi
 else
     set_protocol