New version using /etc/autossh.conf.d/* ssh_config format included files.
- No longer use /opt/autossh/ for *conf files, use /etc/autossh.conf.d/ instead.
- Use default ssh_config file (/etc/ssh/ssh_config).
- ssh_config file is built by including /etc/autossh.conf.d/*.conf files.
- Add default ssh_config files for including.
- Run 'auto' defined SSH host by default.
- Do not define any volume dir.
- Remove obsolete README.txt.