Merge branch 'mtatest'

* mtatest:
  Make an explicit use_smarthost setting
  Consider the empty string as no smarthost
  Setting to undef does not clear the hiera default :(
  undef is the value, Undef the type
  eximconf: drop bad quoting
  the class to include is roles::mta, not mta
  move exim vs. postfix, heavy vs. not, into hiera