Improve server tag schema

server tags are limited with max length and numbers
from nova side. Response schema should strictly verify
those.

This commit improve the server tag schema to have limit
on max number and length.

Change-Id: I71f047d7475d18396c0e154377f2592ec8664b17
diff --git a/tempest/lib/api_schema/response/compute/v2_26/servers.py b/tempest/lib/api_schema/response/compute/v2_26/servers.py
index d873402..b03bdf6 100644
--- a/tempest/lib/api_schema/response/compute/v2_26/servers.py
+++ b/tempest/lib/api_schema/response/compute/v2_26/servers.py
@@ -52,12 +52,7 @@
     'response_body': {
         'type': 'object',
         'properties': {
-            'tags': {
-                'type': 'array',
-                'items': {
-                    'type': 'string'
-                }
-            }
+            'tags': tag_items,
         },
         'additionalProperties': False,
         'required': ['tags']