Do not test the metadata_items limit when there is no limit

Change-Id: I08919f935df1f34e8b75efa2772b08a9eb643872
Closes-Bug: #1401116
diff --git a/tempest/api/compute/servers/test_server_metadata_negative.py b/tempest/api/compute/servers/test_server_metadata_negative.py
index 8b074fd..ee1e652 100644
--- a/tempest/api/compute/servers/test_server_metadata_negative.py
+++ b/tempest/api/compute/servers/test_server_metadata_negative.py
@@ -124,6 +124,9 @@
         # tenant.
         _, quota_set = self.quotas.get_quota_set(self.tenant_id)
         quota_metadata = quota_set['metadata_items']
+        if quota_metadata == -1:
+            raise self.skipException("No limit for metadata_items")
+
         req_metadata = {}
         for num in range(1, quota_metadata + 2):
             req_metadata['key' + str(num)] = 'val' + str(num)