Address a couple of minor pitrtools issues

1. Do not continue archiver's initialization if the queue dir already exists and not empty, warn if it exists and empty. Reported by Ivan Lezhnjov.
2. Change the way configuration files are copied to the standby:
 - add new postgresql_conf_failover and pg_hba_conf_failover for copying configuration files to the failover node. Refuse to continue the failover if those options are not set.
- use postgresql_conf and pg_hba_conf to install configuration files when entering the standby mode.
- fix an issue with no_copy_conf discarding pg_hba.conf.