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.