feat: Add conditional in cheaha.yml for a proxy node

A proxy node need not be a slurm client so we don't run this role

ansible/cheaha.yml

@@ -6,4 +6,4 @@
     - { name: 'cheaha.node', tags: 'cheaha.node' }
     - { name: 'nfs_mounts', tags: 'nfs_mounts' }
     - { name: 'ldap_config', tags: 'ldap_config' }
-    - { name: 'slurm_client', tags: 'slurm_client' }
+    - { name: 'slurm_client', tags: 'slurm_client', when: "'proxy' not in group_names" }