common: send cluster log messages to 'cluster' channel by default

common: send cluster log messages to 'cluster' channel by default

The CLOG_CHANNEL_DEFAULT constant was being abused for two purposes:

 - the default channel to log messages to
 - the name of the config option key in the key/value pair string that is
   used for the default option, e.g. "default=true foo=false bar=false"

Fix this by making the config option key CLOG_CONFIG_DEFAULT_KEY and
replacing throughout, and changing CLOG_CHANNEL_DEFAULT to "cluster" (as
it should be and has been historically).

Fixes: #11177
Signed-off-by: Sage Weil <sage@redhat.com>