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