Fix sftp access on CentOS machines

The sshd_config file that we put everywhere has an invalid value for
where the sftp command is. On RedHat, it's in /usr/libexec - which means
that it is not possible to use SFTP to interact with our CentOS
machines.

Replace the static file with a template so that we can substitute the
correct value based on which distro it is.

Change-Id: Ia9ba88199f4ff024a904431821926dbb26f35ad6