jenkins service: improve curl call in postStart

* Perform HTTP HEAD request instead of full GET (lighter weight)
* Don't log output of curl to the journal (it's noise/debug)
* Use explicit http:// URL scheme
* Reduce poll interval from 10s to 2s (respond to state changes
  quicker). Probably not relevant on boot (lots of services compete for
  the CPU), but online service restarts/reloads should be quicker.
* Pass --fail to curl (should be more robust against false positives)
* Use 4 space indent for shell code.

(cherry picked from commit 78b6e8c319)

nixos/modules/services/continuous-integration/jenkins/default.nix

@@ -164,8 +164,8 @@ in {
       '';
 
       postStart = ''
-        until ${pkgs.curl}/bin/curl -s -L ${cfg.listenAddress}:${toString cfg.port}${cfg.prefix} ; do
-          sleep 10
+        until ${pkgs.curl}/bin/curl -s -L --fail --head http://${cfg.listenAddress}:${toString cfg.port}${cfg.prefix} >/dev/null; do
+            sleep 2
         done
       '';