make letsencryt conditional

debian/README.Debian

@@ -40,6 +40,12 @@ links are added to enable write access to gitlab app
 > shared/cache -> /var/lib/gitlab/cache
 > /usr/share/gitlab/.secret -> /var/lib/gitlab/.secret
 
+8. ssl certificates: This package tries to use letsencrypt package to obtain
+ssl certificates, if it is installed (via Recommends). If letsencrypt is not
+required, you can copy ssl certificate and key to /etc/gitlab/ssl as gitlab.crt
+and gitlab.key. If letsencrypt option is selected, symbolic links are added for
+certificates obtained using letsencrypt to /etc/gitlab/ssl.
+
 Useful diagnostics
 ==================
 

debian/config

@@ -14,10 +14,13 @@ db_go
 db_input high gitlab/ssl || true
 db_go
 
-# Do you want Let's Encrypt?
-db_get gitlab/ssl
-if [ "${RET}" = "true" ]
-then
-	db_input high gitlab/letsencrypt || true
-	db_go
+# Don't prompt for letsencrypt if not installed
+if command -v letsencrypt; then
+  # Do you want Let's Encrypt?
+  db_get gitlab/ssl
+  if [ "${RET}" = "true" ]
+  then
+    db_input high gitlab/letsencrypt || true
+    db_go
+  fi
 fi