Workshop for Dedicated Servers
Dedicated servers can install addons straight from workshop collections, but it requires an addition to the startup parameter. The server will download the collection at startup.
First of all create a collection and find your collection ID. Do note that your collection needs to be public, else, srcds won't be able to access it!
You need a Steam API Key to use this system. You can get one by signing in here. You should keep your key secret.
Applying the command-line parameter
Append the following parameter to the server startup command-line:
+host_workshop_collection WORKSHOPID -authkey YOURKEYGOESHERE
An example could be:
+host_workshop_collection 123456789 -authkey XXXXXF3C2XXXXXXXXX997XX0635XXXXX
The example below starts a dedicated server with 20 player slots. It downloads or updates all of the addons in collection 157384458. This collection includes the Zombie Survival gamemode and a few maps. It sets the gamemode to zombiesurvival and loads one of the downloaded maps.
srcds.exe +maxplayers 20 -console -authkey YOURKEYGOESHERE +host_workshop_collection 157384458 +gamemode zombiesurvival +map zs_cleanoffice_v2
Every time you restart your server it will check that these addons are all up to date.
View this short tutorial to see how it is set up.