[config] cleanup/refactor server sources file values handling
* kill former `sources` method, misnamed since we've only the system source
configuration in the sources file (and some default admin account information)
* introduce new system_source_config and default_admin_config properties to
access to the two information in this file
Then use one or the other when appropriate
import ConfigParser
try:
undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
pass # this conf. file was probably already migrated
else:
config.global_set_option('undo-enabled', bool(undo_actions))