Change checkHNname to checkusername. Fixes #4266.

Test to check that I didn't break the command:

$ crab checkusername
Attempting to extract your username from SiteDB...
Your DN is: /DC=ch/DC=cern/OU=Organic Units/OU=Users/CN=atanasi/CN=710186/CN=Andres Jorge Tanasijczuk
Your username is: atanasi
Finished

I also renamed the getHyperNewsName() function in CRABClient.client_utilities to getUsername().
I also renamed the exception HyperNewsNameException to UsernameException.