Change default ServerKeyBits to 1024

This will set the variable in config file, but existing SSH host keys
are not updated automatically to not surprise the admin with changed
host keys. You can update your SSH host keys by removing existing keys
and issuing command 'dpkg-reconfigure openssh-server'.

Related links:
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=727622