Partially revert "Drop ostree-grub2" for ppc64le

This partially reverts commit fb1e4b0
for ppc64le only since we still use Anaconda + `grub2-mkconfig` there.

See: coreos#173

bootable-rpm-ostree.yaml

@@ -17,7 +17,7 @@ packages-aarch64:
 packages-armhfp:
   - extlinux-bootloader
 packages-ppc64le:
-  - grub2
+  - grub2 ostree-grub2
 packages-s390x:
   - s390utils-base
 packages-x86_64: