Revert the workarouds for kernel override

Centos 7.5 is released with the new kernel so workaround applied
for Centos 7.4 update kernel needs revert.
The patch reverts the following 4 commits applied
in order to override kernel. Some nested VMs(Skylake) have
issues running virt-customize with kvm backend, so set
LIBGUESTFS_BACKEND_SETTINGS=force_tcg to not use
kvm accelaration. Setting this option drops performance
for running virt-customize a little but can't find any other
option until it's fixed in kernel/kvm.

Revert "add option to turn on/off non default kernel"

This reverts commit fec6fd069d.

Revert "Do not attempt install kernel when chrooted"

This reverts commit d991c1033f.

Revert "Ensure libguestfs_kernel_override kernel is installed"

This reverts commit ff0a5c9ac7.

Revert "Set SUPERMIN_* variables to working kernel"

This reverts commit 6aac21081d.

Change-Id: If46010d7ca14f9dde9a49173aa0b6de91c3826a8
Related-Bug: #1743749

roles/modify-image/defaults/main.yml

@@ -27,8 +27,3 @@ modify_image_vc_trace: false
 libguestfs_mode: true
 # TODO(sshnaidm): implement initramfs image modifying
 initramfs_image: false
-
-# kernel bug
-# https://bugzilla.redhat.com/show_bug.cgi?id=1535973
-libguestfs_kernel_override: 3.10.0-693.el7.x86_64
-use_non_default_kernel: true

roles/modify-image/tasks/libguestfs.yml

@@ -37,17 +37,6 @@
   set_fact: _vc_args="{{ _vc_args }} --root-password password:{{ modify_image_vc_root_password }}"
   when: modify_image_vc_root_password is defined
 
-# kernel bug
-# https://bugzilla.redhat.com/show_bug.cgi?id=1535973
-- name: ensure libguestfs_kernel_override kernel is installed
-  yum:
-    name: "kernel-{{ libguestfs_kernel_override }}"
-    state: present
-  become: true
-  when:
-    - not chrooted|bool
-    - use_non_default_kernel|bool
-
 - name: Run virt-customize on the provided image
   shell: >
     virt-customize {{ _vc_args }}
@@ -55,11 +44,9 @@
     > {{ working_dir }}/{{ modify_script|default('modify_image')|replace('/','_') }}.log 2>&1
   environment:
     LIBGUESTFS_BACKEND: direct
-    # FIXME(ykarel) SUPERMIN_* can be removed once
+    # FIXME(ykarel) LIBGUESTFS_BACKEND_SETTINGS can be removed once
     # https://bugs.launchpad.net/tripleo/+bug/1743749 is completely fixed.
-    SUPERMIN_KERNEL_VERSION: "{{ libguestfs_kernel_override }}"
-    SUPERMIN_KERNEL: "/boot/vmlinuz-{{ libguestfs_kernel_override }}"
-    SUPERMIN_MODULES: "/lib/modules/{{ libguestfs_kernel_override }}"
+    LIBGUESTFS_BACKEND_SETTINGS: force_tcg
   args:
     chdir: "{{ modify_image_working_dir }}"