Add username to uploaded log file url. 

1. In client_utilities::getUsernameFromSiteDB(), convert retrieved username from SiteDB from 'null' to None.
2. In checkusername command, if username retrieved from SiteDB is None (because of 1., this includes username = 'null', and I think is actually the only case), give an appropriate error message. Before we were printing for example "Username is: null".
3. Add username to the upload log file url returned to the user.

Requires https://github.com/dmwm/WMCore/pull/5513.