Consolidate centos/fedora/redhat required_defaults

There's very little difference in the package lists. I'm just
guessing there, but I believe g++ and xml2/xslt devel packages are
for working with libvirt, and the python3-selinux either comes
pre-installed or is installed at some earlier point by other
playbooks.

Change-Id: I5865c65cae695491524a8828c8abb48393a197a0

Author: eb4x

playbooks/roles/bifrost-create-vm-nodes/defaults/required_defaults_CentOS.yml

@@ -12,6 +12,8 @@ required_packages:
  - libvirt-daemon
  - libvirt-daemon-kvm
  - python3-libvirt
+ - python3-libselinux
  - libxslt-devel
  - libxml2-devel
  - edk2-ovmf
+test_vm_emulator: "/usr/libexec/qemu-kvm"