Merge LibvirtBridgeDriver class into LibvirtGenericVIFDriver

This introduces the LibvirtGenericVIFDriver class which uses
the 'vif_type' mapping field to determine what type of host
network configuration is required.

The LibvirtBridgeDriver class functionality is merged into
LibvirtGenericVIFDriver. For backwards compatibility with
the Folsom release, the existing LibvirtBridgeDriver class
is made to inherit from LibvirtGenericVIFDriver and directly
call the bridge specific setup APIs. This eases migration to
the new VIF impl during the Grizzly deployment lifecycle, with
an expectation that the LibvirtBridgeDriver stub will be
deleted in the Hxxxxx release.

DocImpact
Blueprint: libvirt-vif-driver
Change-Id: I948158b5ee0a97f666bb4d21020b552d3475b6d3
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Merge LibvirtBridgeDriver class into LibvirtGenericVIFDriver"