[lib] kadeploy gen: allow specifying custom kexec script

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

@@ -24,7 +24,7 @@ trusted_deployment:
 <% end %>
 kexec:
   server_precmd: "true"
-  script: /etc/kadeploy3<%= suffix %>/kexec.sh
+  script: /etc/kadeploy3<%= suffix %>/<%= data.fetch('kexec_script', 'kexec.sh') %>
   <% if ! data['partitioning.disable_swap'].nil? %>disable_swap: <%= data['partitioning.disable_swap'] %><% end %>
 timeouts:
   reboot: <%= data['reboot.timeout'] %>