iscsigw: install python-requests

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>
(cherry picked from commit 167737dd3de02057403fb458c50d22cf94a85b95)