[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
from yams import schema2sql as y2sql
dbhelper = repo.system_source.dbhelper
rdefdef = schema['CWSource'].rdef('name')
attrtype = y2sql.type_from_constraints(dbhelper, rdefdef.object, rdefdef.constraints).split()[0]
cursor = session.cnxset['system']
sql('UPDATE entities SET asource = source WHERE asource is NULL')
dbhelper.change_col_type(cursor, 'entities', 'asource', attrtype, False)
dbhelper.change_col_type(cursor, 'entities', 'source', attrtype, False)