Thanks for ordering OpenSprinkler.
To answer your question about relay: the one you linked to seems to be a DC relay (specifically the spec says Coil Power is 4 watts DC). I suspect it’s not compatible with OpenSprinkler, because DC relay coils will probably not work well under AC voltage. In the past I’ve used this 24VAC relay with OpenSprinkler:
http://www.mouser.com/ProductDetail/TE-Connectivity/RT314524/?qs=%2fha2pyFaduiqgdqtVWfjKo5IK6OgllgyKqIUN%252b%252bDeJ5M3AT%2fmg8vzw%3d%3d
which has worked well. What to look for here is that the coil voltage should be 24VAC (or anything between 22VAC to 28VAC). Of course you should also check the contact rating is sufficient to drive your lights (maximum current, maximum voltage). The one above can switch up to 16 amp and up to 250VAC, which I assume is sufficient for your application.
An alternative solution, which is what I really prefer, is to use a remote power socket, which is low-cost, a lot safer, and requires almost no wiring. The latest OpenSprinkler firmware (2.1.1) has added built-in support for radio frequency power sockets. This is a better solution if you want to switch multiple lights or multiple devices, because it involves no additional wiring from OpenSprinkler. For details you can check this blog post: https://opensprinkler.com/opensprinkler-firmware-2-1-1-new-feature-control-remote-power-sockets/