Use expect_empty_body flag

The create_router_on_l3_agent and add_dhcp_agent_to_network API
expects to get a response without the body so their tests must
use expect_empty_body flag.

This patch also makes the body to be None because now "null" is passed
as body. Empty string will be passed once the issue is fixed[1] and
then the setting will be not needed.

[1]: https://review.openstack.org/#/c/291545/

Change-Id: If33f518b817f983a107a736172026d86c47f9878
Related-Bug: #1555921
diff --git a/tempest/lib/services/network/agents_client.py b/tempest/lib/services/network/agents_client.py
index c5d4c66..9bdf090 100644
--- a/tempest/lib/services/network/agents_client.py
+++ b/tempest/lib/services/network/agents_client.py
@@ -44,7 +44,7 @@
         # link to api-site.
         # LP: https://bugs.launchpad.net/openstack-api-site/+bug/1526670
         uri = '/agents/%s/l3-routers' % agent_id
-        return self.create_resource(uri, kwargs)
+        return self.create_resource(uri, kwargs, expect_empty_body=True)
 
     def delete_router_from_l3_agent(self, agent_id, router_id):
         uri = '/agents/%s/l3-routers/%s' % (agent_id, router_id)
@@ -65,4 +65,4 @@
         # link to api-site.
         # LP: https://bugs.launchpad.net/openstack-api-site/+bug/1526212
         uri = '/agents/%s/dhcp-networks' % agent_id
-        return self.create_resource(uri, kwargs)
+        return self.create_resource(uri, kwargs, expect_empty_body=True)
diff --git a/tempest/lib/services/network/base.py b/tempest/lib/services/network/base.py
index 620e0f1..b6f9c91 100644
--- a/tempest/lib/services/network/base.py
+++ b/tempest/lib/services/network/base.py
@@ -63,6 +63,8 @@
         # body. Otherwise we returns the body as it is.
         if not expect_empty_body:
             body = json.loads(body)
+        else:
+            body = None
         self.expected_success(201, resp.status)
         return rest_client.ResponseBody(resp, body)
 
@@ -75,5 +77,7 @@
         # body. Otherwise we returns the body as it is.
         if not expect_empty_body:
             body = json.loads(body)
+        else:
+            body = None
         self.expected_success(200, resp.status)
         return rest_client.ResponseBody(resp, body)