Avoid empty string for length in os-getConsoleOutput

If the caller does not pass in any value for length, we should
not be sending length='' to the os-getConsoleOutput REST API
as Nova v2 API does not handle it properly

Change-Id: I61db6a92418935dd4d0a8fc149d18417bf434b94
Closes-Bug: #1329563
Merge "Avoid empty string for length in os-getConsoleOutput"