Add comment in manage_unit deployed files

It's useful to see clearly see that files deployed with puppet
are deployed with puppet.

Add a comment to top of files deployed by `systemd::manage_unit`
and  `systemd::manage_dropin`.

spec/defines/manage_dropin_spec.rb

@@ -17,6 +17,9 @@
             }
           end
 
+          it { is_expected.to compile.with_all_deps }
+          it { is_expected.to contain_systemd__dropin_file('foobar.conf').with_content(%r{^# Deployed with puppet$}) }
+
           context 'setting some parameters simply' do
             let(:params) do
               super().merge(

templates/unit_file.epp

@@ -4,6 +4,7 @@
  Optional[Hash] $install_entry,
  Optional[Hash] $timer_entry,
 | -%>
+# Deployed with puppet
 <% if $unit_entry { -%>
 
 [Unit]