Fix url in list_services

This commit fixes list_services by changing the url passed to self.get
from 'services' to url, because url can concatenate **params
to 'services'.

Change-Id: I2e2ebb72732ab95d5f9c1d988037c5e263bf2a71
diff --git a/tempest/lib/services/identity/v3/services_client.py b/tempest/lib/services/identity/v3/services_client.py
index 17b0f24..7bbe850 100644
--- a/tempest/lib/services/identity/v3/services_client.py
+++ b/tempest/lib/services/identity/v3/services_client.py
@@ -76,7 +76,7 @@
         url = 'services'
         if params:
             url += '?%s' % urllib.urlencode(params)
-        resp, body = self.get('services')
+        resp, body = self.get(url)
         self.expected_success(200, resp.status)
         body = json.loads(body)
         return rest_client.ResponseBody(resp, body)