Allow to update tftpd configuration

This patch reworks tftpd-hpa module to allow specify tftpd
configuration options like:
  username, directory, options

Change-Id: I4fbaa9a4149ca230008099aa29834a8f9de6449b
diff --git a/README.rst b/README.rst
index 3e155d6..e741ac8 100644
--- a/README.rst
+++ b/README.rst
@@ -17,6 +17,11 @@
     tftpd_hpa:
       server:
         enabled: true
+        username: 'tftp'
+        path: '/var/lib/tftproot'
+        options:
+          - secure
+          - map-file: '/var/lib/tftproot/map-file'
 
 
 More information