Added and modified distributor custom info tests

1. Changes to update the max length of keynames and values to 255. 
2. Added tests for add/update/remove custom info using uuid
3. Custom info remove and update bound to fail due to BZ: 
https://bugzilla.redhat.com/show_bug.cgi?id=973929
https://bugzilla.redhat.com/show_bug.cgi?id=990299