Add spec for postfix::mta, do not include postfix anymore

spec/classes/postfix_mta_spec.rb

+require 'spec_helper'
+
+describe 'postfix::mta' do
+  let (:facts) { {
+    :needs_postfix_class_with_params => true,
+    :osfamily                        => 'Debian',
+  } }
+
+  it { should contain_postfix__config('mydestination').with_value('bar') }
+  it { should contain_postfix__config('mynetworks').with_value('baz') }
+  it { should contain_postfix__config('relayhost').with_value('foo') }
+end