install-server::tftp-server cleanups

- Set backup => false: do not filebucket debian-installer binaries;
  they're large and it's also useless to back them up.
- Drop /srv/tftpboot/restricted (it's empty) & its resepective lighttpd
  configuration.
- Drop /tftpboot symlink, it's useless and non-FHS compliant.
- ensure => s/latest/ensure/ for atftpd.
- Remove comment that states that atftpd is started by inetd; simply
  untrue, as our atftpd-default has USE_INETD=false.
- Add a Service['atftpd'] stanza and notify it from the config File
  resource.
- Indent

Change-Id: I5999c75e9eaa912026b2cd21221d80d8932ff02c