Compatibility with older python versions

In older python versions, Exception is not a subclass of object.
Make certificate_request on slightly broken ilo's work

Some of my ilo's return an empty CSR. Make hpilo.py not crash.
Make process_info_tag2 work again
Don't break on empty certificate signing requests
certificate_signing_request is special

It's the only mode=write tag that is actually supposed to return
something. Special-case it in hpilo_cli and _control_tag so that
actually happens and is usable in delayed calls.
Release bugfix version 2.0.1