convert virt drivers to fully dynamic loading
convert virt drivers to fully dynamic loading

Progress on bp:virt-driver-cleanup

This series of patches converts the virt drivers to loading via
importutils making it possible to add a virt driver without changing
core code. It deprecates the use of connection_type in favor of a
full driver name in compute_driver.

Based on email thread with Vish and Jay Pipes on approaches to do
this.

Change-Id: I48366ec7efc7b095859988e5e6ac5a853b98b8a7

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects