[rhel]Set kernel argument rd.multipath=default

This is required to boot in some multipath environments, and should have
no effect in non-multipath environments.

Since there is always a non-empty DIB_BOOTLOADER_DEFAULT_CMDLINE now,
the placeholder argument and TODO is removed.

Author: steveb

images/edpm-hardened-uefi-rhel-9.yaml

@@ -29,7 +29,5 @@
     DIB_DHCP_TIMEOUT: '60'
     DIB_IMAGE_SIZE: '5'
     DIB_MODPROBE_BLACKLIST: 'usb-storage cramfs freevxfs jffs2 hfs hfsplus squashfs udf bluetooth'
-    # TODO remove memtest=0 when this change is available:
-    # https://review.opendev.org/c/openstack/diskimage-builder/+/884644
-    DIB_BOOTLOADER_DEFAULT_CMDLINE: 'memtest=0'
+    DIB_BOOTLOADER_DEFAULT_CMDLINE: 'rd.multipath=default'
     DIB_BOOTLOADER_VIRTUAL_TERMINAL: ''