Config: fix the mess with channels

There is no longer config.channels.  Instead, use config.get_channel_urls() as
before.  This is because the channel urls must be computed from other
configuration values (namely, base_urls and subdir).

I also fixed the test (it was pointing to the wrong condarc).
Expose and use default and rc only configuration
'system' channel actually just needs to refer to config, not use config_rc