Add purge_config option for all configs

Adding a purge_config option for all configs (specifically left out
services/lbaas.pp and services/vpnaas.pp as they require default
entries). Adding this option for ease of management and readability.
If users wish, they can use this to ensure that only the options they
are setting make it into the configs.
  *rebased my changes

Change-Id: I2286acb350ae6300f2e4c8dd320f2534ddf09bf9

spec/classes/neutron_agents_dhcp_spec.rb

@@ -19,7 +19,8 @@ describe 'neutron::agents::dhcp' do
       :dhcp_driver              => 'neutron.agent.linux.dhcp.Dnsmasq',
       :root_helper              => 'sudo neutron-rootwrap /etc/neutron/rootwrap.conf',
       :enable_isolated_metadata => false,
-      :enable_metadata_network  => false }
+      :enable_metadata_network  => false,
+      :purge_config             => false }
   end
 
   let :test_facts do
@@ -77,6 +78,12 @@ describe 'neutron::agents::dhcp' do
       is_expected.to contain_service('neutron-dhcp-service').that_subscribes_to('Package[neutron]')
     end
 
+    it 'passes purge to resource' do
+      is_expected.to contain_resources('neutron_dhcp_agent_config').with({
+        :purge => false
+      })
+    end
+
     context 'with manage_service as false' do
       before :each do
         params.merge!(:manage_service => false)