Support custom keyfile paths

This makes it possible to use resource_record with "standalone"
keyfiles by introducing a keyfile parameter

lib/puppet_bind/provider/nsupdate.rb

@@ -51,6 +51,8 @@ module PuppetBind
         file.close
         if keyed?
           nsupdate('-y', tsig_param, file.path)
+        elsif keyfile?
+          nsupdate('-k', kfile, file.path)
         else
           nsupdate(file.path)
         end
@@ -81,6 +83,14 @@ module PuppetBind
         resource[:keyname]
       end
 
+      def kfile
+        resource[:keyfile]
+      end
+
+      def keyfile?
+        !kfile.nil?
+      end
+
       def hmac
         resource[:hmac]
       end