This is useful if you want to add things like net.ifnames=0 (to make
sure your undercloud has eth* named nics) or
mitigations=off for that extra performance.
Tested by adding:
undercloud_extra_grub_params: net.ifnames=0 mitigations=off
And observing the following on the undercloud:
(undercloud) [stack@undercloud ~]$ cat /proc/cmdline
console=ttyS0 root=/dev/vda net.ifnames=0 mitigations=off
Change-Id: Ie814765897699705a2826ed1a8c8d8f85d579028
1e9167d949