Use distro provided GRUB efi instead of creating it like ironic GRUB
setup doc[1]. This avoids below ubuntu ironic-python-agent images PXE
booting failure.
----------
error: invalid magic number.
error: you need to load the kernel first.
Press any key to continue...
----------
This also fixes x86_64 uefi pxe booting issue by setting up GRUB efi
for x86_64.
Besides, GRUB setup only needs to do once at bootstrap stage.
[1]:
https://docs.openstack.org/ironic/train/install/configure-pxe.html#uefi-pxe-GRUB-setup
Closes-Bug: #1879265
Change-Id: I8be5bdf5f1a62751aefe6bd0959e8f558fcfe591