iscsigw: install python-requests

Typical error at rbd-target-api startup:

```
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: Traceback (most recent call last):
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: File "/usr/bin/rbd-target-api", line 39, in <module>
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: from gwcli.utils import (APIRequest, valid_gateway, valid_client,
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: File "/usr/lib/python2.7/site-packages/gwcli/utils.py", line 1, in <module>
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: import requests
Sep 25 12:12:29 iscsi-gw0 rbd-target-api[9959]: ImportError: No module named requests
```

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

roles/ceph-iscsi-gw/tasks/non-container/prerequisites.yml

@@ -63,7 +63,9 @@
         state: "{{ (upgrade_ceph_packages|bool) | ternary('latest','present') }}"
       register: result
       until: result is succeeded
-      with_items: "{{ ceph_iscsi_pkgs }}"
+      with_items:
+        - "{{ ceph_iscsi_pkgs }}"
+        - python-requests
 
 - name: check the status of the target.service override
   stat: