CentOS 8: seed VM & bifrost

* Change default seed VM image to CentOS 8
* Change default bifrost deploy image to CentOS 8
* Workaround DIB bug
  https://bugs.launchpad.net/diskimage-builder/+bug/1866847 by setting
  DIB_DISABLE_KERNEL_CLEANUP to 1
* Install iptables on seed for SNAT - missing on CentOS 8
* Fix provider network MTU lookup for empty string
* Bump stackhpc.libvirt-host to 1.7.0 for CentOS 8 support
* Bump stackhpc.libvirt-vm to 1.13.0 for CentOS 8 support
* Bump jriguera.configdrive for Python 3 support

Change-Id: Ie0edf6a924a914395c6502e2d5cf1139bce14a48
Story: 2006574
Task: 39000