Convert get_console_output length=None to -1 for Nova V3 API calls

Nova commit 8e5577f52f660960738f1fc0384dff77fa30fc8a makes the nova v3
get_console_output extension handle unlimited length with -1.

In the nova v2 API the length parameter is optional but the nova v3 API
uses jsonschema validation so it expects a value to check against.

Closes-Bug: #1328582

Change-Id: Ib1ae85e644e08c24aaf0ba2baaa8812286ef0ebd
diff --git a/tempest/services/compute/v3/json/servers_client.py b/tempest/services/compute/v3/json/servers_client.py
index f397c4b..026ad2c 100644
--- a/tempest/services/compute/v3/json/servers_client.py
+++ b/tempest/services/compute/v3/json/servers_client.py
@@ -431,6 +431,9 @@
         return self.action(server_id, 'shelve_offload', None, **kwargs)
 
     def get_console_output(self, server_id, length):
+        if length is None:
+            # NOTE(mriedem): -1 means optional/unlimited in the nova v3 API.
+            length = -1
         return self.action(server_id, 'get_console_output', 'output',
                            common_schema.get_console_output, length=length)