Permit using different libvirt hypervisors.

Folk will want to deploy lxc/qemu etc overclouds. The libvirt type
needs to be controllable for this, independent of software
installation.

Change-Id: I29e5fea72f9cb983576410baa8f50ab060c08fe3
Remove deprecated and unused nova setting.

Change-Id: I01a7c0b5faac0338594988b2e69981aad215cb33