Upstream ironic went from $net_default_ip to $net_default_mac in
ironic/drivers/modules/master_grub_cfg.txt with
https://review.opendev.org/#/c/578959/

This commit makes the same change for
ansible/roles/ironic/templates/ironic_pxe_uefi.default.j2

Using $net_default_ip breaks ironic standalone deployments with
[dhcp]dhcp_provider = none

Change-Id: I2ca9a66d2bdb0aab5cd9936c8be8206e6ade3bd5
Closes-Bug: 1842078