chore(gerrit): offer projects configuration and factor out private SSH keys

Previously, we needed to hardcode the URL for private SSH keys,
this is cleaned up and we can iterate over each project for its
configuration.

Configuration is at deployment time.

Signed-off-by: Raito Bezarius <raito@lix.systems>

nix/coordinator.nix

@@ -174,6 +174,14 @@ in
           '';
           example = [ "lix" ];
         };
+
+        projects = lib.mkOption {
+          type = lib.types.listOf lib.types.str;
+          description = ''
+            List of projects which are to check on Gerrit.
+          '';
+          example = [ "lix" ];
+        };
       };
 
       binaryCache = {