Use safe shim binary paths on redhat family

The path /boot/efi/EFI/BOOT/BOOTX64.EFI is probably the packaged
signed shim binary but it could be any bootloader binary from another
OS on a dual-boot system.

The shim packages also place the same binary at
/boot/efi/EFI/<distro>/shimx64.efi, so this change switches to using
that path instead.

Change-Id: I450af490ee149a268cd5fb6727010834262de1f6

playbooks/roles/bifrost-ironic-install/defaults/required_defaults_CentOS.yml

@@ -1,3 +1,4 @@
 ---
 efi_distro: centos
 grub_efi_binary: /boot/efi/EFI/centos/grubx64.efi
+shim_efi_binary: /boot/efi/EFI/centos/shimx64.efi

playbooks/roles/bifrost-ironic-install/defaults/required_defaults_Fedora.yml

@@ -7,6 +7,7 @@ mysql_service_name: mariadb
 tftp_service_name: tftp
 efi_distro: fedora
 grub_efi_binary: /boot/efi/EFI/fedora/grubx64.efi
+shim_efi_binary: /boot/efi/EFI/fedora/shimx64.efi
 required_packages:
   - dnsmasq
   - firewalld

playbooks/roles/bifrost-ironic-install/defaults/required_defaults_RedHat.yml

@@ -1,3 +1,4 @@
 ---
 efi_distro: rhel
 grub_efi_binary: /boot/efi/EFI/rhel/grubx64.efi
+shim_efi_binary: /boot/efi/EFI/rhel/shimx64.efi

playbooks/roles/bifrost-ironic-install/defaults/required_defaults_RedHat_family.yml

@@ -6,7 +6,6 @@ ipxe_efi_binary: ipxe-x86_64.efi
 ironic_rootwrap_dir: /usr/bin/
 mysql_service_name: mariadb
 tftp_service_name: tftp
-shim_efi_binary: /boot/efi/EFI/BOOT/BOOTX64.EFI
 required_packages:
   - mariadb-server
   - dnsmasq