[gen/kadeploy] place custom variable to use when using tftp instead of http

Coté ref repo, il faut juste définir "force_tftp" à false par défaut et le placer à true pour les cluster où on veut le forcer

--- a/generators/kadeploy/kadeployg5k-dev.yaml
+++ b/generators/kadeploy/kadeployg5k-dev.yaml
   defaults:
  ...
+  force_tftp: false
   grat:
  ...
+    force_tftp: true

ce qui génère:

--- a/modules/generated/files/grid5000/kadeploy/server_dev/nancy/grat-cluster.conf
+++ b/modules/generated/files/grid5000/kadeploy/server_dev/nancy/grat-cluster.conf
@@ -97,3 +97,5 @@ automata:
     - timeout: 900
       type: HardReboot
       retries: 0
+custom_variables:
+  FORCE_TFTP="1"

lib/refrepo/gen/puppet/templates/kadeployg5k.conf.erb

@@ -128,3 +128,7 @@ automata:
     - timeout: <%= data['boot.timeout.1'] %>
       type: HardReboot
       retries: 0
+<%- if data['force_tftp'] -%>
+custom_variables:
+  FORCE_TFTP="1"
+<%- end -%>