Add assert for return code

* Add assertion that reposne was sucess
  before process response.text()
* Add header to request to avoid 406 error

Change-Id: If41598e8c1ef5d9bf36847a750008d1203b4ed84
Closes-Prod: PROD-26423
(cherry picked from commit 30bd90c986234343aabf03ec5f174026d02d4988)
diff --git a/cvp-sanity/cvp_checks/tests/test_stacklight.py b/cvp-sanity/cvp_checks/tests/test_stacklight.py
index 1ffb118..ec6ed40 100644
--- a/cvp-sanity/cvp_checks/tests/test_stacklight.py
+++ b/cvp-sanity/cvp_checks/tests/test_stacklight.py
@@ -65,14 +65,20 @@
         expr_form='pillar')
 
     IP = salt_output.values()[0]
+    headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
     proxies = {"http": None, "https": None}
-    resp = json.loads(requests.post('http://{0}:9200/log-{1}/_search?pretty'.
-                                    format(IP, today),
-                                    proxies=proxies,
-                                    data='{"size": 0, "aggs": '
-                                         '{"uniq_hostname": '
-                                         '{"terms": {"size": 500, '
-                                         '"field": "Hostname.keyword"}}}}').text)
+    data = ('{"size": 0, "aggs": '
+            '{"uniq_hostname": '
+            '{"terms": {"size": 500, '
+            '"field": "Hostname.keyword"}}}}')
+    response = requests.post(
+        'http://{0}:9200/log-{1}/_search?pretty'.format(IP, today),
+        proxies=proxies,
+        headers=headers,
+        data=data)
+    assert 200 == response.status_code, 'Unexpected code {}'.format(
+        response.text)
+    resp = json.loads(response.text)
     cluster_domain = local_salt_client.cmd('salt:control',
                                            'pillar.get',
                                            ['_param:cluster_domain'],