update: add missing hosts in facts gathering

iscsigws were missing.
The 'complete upgrade' couldn't complete because rolling_update was set
to False for iscsigw nodes.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

infrastructure-playbooks/rolling_update.yml

@@ -48,6 +48,7 @@
     - "{{ rbd_mirror_group_name|default('rbdmirrors') }}"
     - "{{ nfs_group_name|default('nfss') }}"
     - "{{ client_group_name|default('clients') }}"
+    - "{{ iscsi_gw_group_name|default('iscsigws') }}"
 
   become: True
   gather_facts: False