common.yaml

environments:
  - production:
      environment: production
      modulepath: /etc/puppet/modules:/etc/puppet-modules
      manifest: $manifestdir/site.pp
  - testing:
      environment: testing
      modulepath: /etc/puppet/modules:/etc/puppet-environment/testing
      manifest: $manifestdir/site.pp

puppet.conf.erb

<% if environments != 'UNDEFINED'%>
<% environments.each do |env| %>
<% env.each do |key,value| %>
[<%= value['environment'] %>]
    <%- if value.has_key?("manifestdir") -%>
    manifest=<%= value['manifestdir'] %>
    <%- end -%>
    <%- if value.has_key?("manifest") %>
    manifest=<%= value['manifest'] %>
    <% end -%>
    <%- if value.has_key?("modulepath") %>
    modulepath=<%= value['modulepath'] %>
    <% end -%>
    <%- if value.has_key?("templatedir") %>
    templatedir=<%= value['templatedir'] %>
    <% end -%>
<% end %>    
<% end %>
<% end %>