ic: Mark binary_swap_gpdb as optional input for resgroup jobs.

binary_swap_gpdb was input of all resgroup jobs, but as it's not `get` by the resgroup sles job an error will be triggered:

    missing inputs: binary_swap_gpdb

Fixed by marking it as optional.

concourse/tasks/ic_gpdb_resgroup.yml

@@ -10,6 +10,7 @@ inputs:
   - name: ccp_src
   - name: cluster_env_files
   - name: binary_swap_gpdb
+    optional: true
 outputs:
 
 params: