[generator] [kadeploy] Allow to configure the install_bootloader script

Caution: we set no default value, so the g5k configuration (in
g5k-platform-production) needs to be updated!

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

@@ -42,7 +42,7 @@ remoteops:
   - name: very_hard
     cmd: <%= data['remoteops.power_off.very_hard.cmd'] %><%end %>
 boot:
-  install_bootloader: /etc/kadeploy3<%= suffix %>/install_grub2.sh
+  install_bootloader: /etc/kadeploy3<%= suffix %>/<%= data['deploy.install_bootloader'] %>
   kernels:<%
 kernel_prod = 'kernels/kadeploy3-deploy-kernel-buster.vmlinuz'
 initrd_prod = 'kernels/kadeploy3-deploy-kernel-buster.initrd.img'