Add interface-names to Rhel8 images

With EL8, net.ifnames is set to 0 in the grub config. This is a change
in the default image which can have user impacts, Details in bug[1].

We already added "interface-names" to centos8 images[2]. With this
change we are adding "interface-names" to remove net.ifnames=0 from
rhel8 image as well.

[1] https://bugs.launchpad.net/tripleo/+bug/1841441
[2] https://review.opendev.org/#/c/712571/

Change-Id: I91567185464997eb77217b14e7efb97cbecf6e77

image-yaml/overcloud-hardened-images-rhel8.yaml

@@ -5,3 +5,5 @@ disk_images:
     distro: rhel
     environment:
       DIB_RELEASE: '8'
+    elements:
+      - interface-names

image-yaml/overcloud-images-ceph-rhel8.yaml

@@ -4,3 +4,5 @@ disk_images:
     type: qcow2
     distro: rhel
     release: 8
+    elements:
+      - interface-names

image-yaml/overcloud-images-rhel8.yaml

@@ -6,6 +6,7 @@ disk_images:
     release: 8
     elements:
       - selinux-permissive
+      - interface-names
     environment:
       FS_TYPE: xfs
     options:
@@ -17,3 +18,4 @@ disk_images:
     release: 8
     elements:
       - selinux-permissive
+      - interface-names