Correctly load up2date configuration

Previously, a configuration file name of None was being passed into
up2dateInitConfig().  This resulted in a correct configuration import,
but failed to properly save the configuration back to disk in the event
a different serverURL was supplied.  This change removes support for
customizing the up2date filename entirely, and relies on up2date to
choose the default config filename.
Merge pull request #3332 from jlaska/rhn_register_cfgfile_fix

rhn_register - Correctly load up2date configuration