It appears that some Ubuntu 14.04 images only contain the base kernel package
and leave out the extra drivers package, linux-image-extra-$(uname -r).
If this package is not installed, openstack-ansible playbooks will fail on
modprobing kernel modules that are not installed (i.e. vhost_net.ko).
linux-image-extra-virtual should provide the linux-image-extra package for
currently installed base kernel package.
Change-Id: If2f008357117495c9e4b80dbfef68503fa93a809
Closes-Bug: 1595323
27c0ee1e47