refactor dbinterface

[mpavlase]

at least show real cause of failures, when connection to DB couldn't be established

• use psycopg2.connect as context manager
• logger as global variable not as class attribute
• keep dryrun (execution of query within transaction [commit/rollback])
• keep all parms in query()
• add get_config_file()
• del most of try and except
• always log messages set level according to the bug (get rid of if debug: logger.debug(...))