salt: Set roster to kubernetes directly in Salt master config

Add roster directly in salt master config so that it's simpler to do
some manual call to `salt-ssh` directly.

Also in Salt 3002.5 `roster` from kwargs in ssh command is ignored,
sees saltstack/salt#59657

salt/metalk8s/salt/master/files/master-99-metalk8s.conf.j2

@@ -29,6 +29,9 @@ roster_defaults:
     use_superseded:
       - module.run
 
+# Set default roster to kubernetes
+roster: kubernetes
+
 # Globally disable SSH host key checks
 no_host_keys: true