Allow to send optional values to configuration

Under certain situations, is useful to send some
extra configuration to pip.conf, such as the retries,
timeout, etc...
This change allows to pass extra values as optional
settings to the template.

Change-Id: Ibcf431a17717fb7f8b8f501603f035fd9456def4

manifests/init.pp

@@ -4,6 +4,7 @@ class pip (
   $index_url = 'https://pypi.python.org/simple',
   $trusted_hosts = [],
   $manage_pip_conf = false,
+  $optional_settings = {},
 ) {
   include pip::params
   validate_array($trusted_hosts)

templates/pip.conf.erb

@@ -6,3 +6,7 @@ trusted-host =
   <%= trusted_host %>
 <% end -%>
 <% end -%>
+
+<% @optional_settings.keys.sort.each do |setting| -%>
+<%= setting -%> = <%= @optional_settings[setting] %>
+<% end -%>