Expose option to set libvirt nic driver.

This commit adds an environment variable, $LIBVIRT_NIC_DRIVER, that
can be set to override the default e1000 driver.  Using the virtio
libvirt driver for network interfaces results in much better network
performance between vm's.  Local testing indicates the disk deployment
by nova-baremetal-deploy-helper going from around 4 minutes to 1
minute after switching to virtio.

virtio may be unstable on Ubuntu, see comments in
https://bugzilla.redhat.com/show_bug.cgi?id=910619 for more details.

Change-Id: I4af35be6dd96fbd3201adac25a6e03bf5ecc712d
Merge "Expose option to set libvirt nic driver."