Use optional_commands for Puppet 2.6 compatibility

Without this, using the module with puppet 2.6 results in the
error: "Could not find a default provider for vs_bridge"

Fix suggested by Dan Bode via email

lib/puppet/provider/vs_port/ovs.rb

@@ -1,7 +1,7 @@
 require "puppet"
 
 Puppet::Type.type(:vs_port).provide(:ovs) do
-  commands :vsctl => "/usr/bin/ovs-vsctl"
+  optional_commands :vsctl => "/usr/bin/ovs-vsctl"
 
   def exists?
     vsctl("list-ports", @resource[:bridge]).include? @resource[:interface]