fix postfix::satellite class

fix postfix::satellite class

i got this error using postfix::satellite:

  Error: Could not retrieve catalog from remote server: Error 400 on
  SERVER: Cannot reassign variable valid_fqdn at
  /etc/puppet/modules/postfix/manifests/satellite.pp:32 …

reassigning a variable within a given scope doesn't work in puppet
see https://docs.puppetlabs.com/puppet/latest/reference/lang_variables.html#no-reassignment

i still think overriding $::fqdn is not a good idea, but for now,
i just want to get this class working again.