Change checkHNname to checkCERNusername.

Test to check that I didn't break the command:

$ crab checkCERNusername
Attempting to extract your CERN username from SiteDB...
Your DN is: /DC=ch/DC=cern/OU=Organic Units/OU=Users/CN=atanasi/CN=710186/CN=Andres Jorge Tanasijczuk
Your CERN username is: atanasi
Finished

I also tested the renamed getCERNUsername function in my CRAB config and it worked.