Add linux-image-extra-virtual to host packages

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

vars/ubuntu-14.04.yml

@@ -66,6 +66,7 @@ openstack_host_packages:
   - irqbalance
   - libkmod-dev
   - libkmod2
+  - linux-image-extra-virtual
   - lvm2
   - python-software-properties
   - python-dev

vars/ubuntu-16.04.yml

@@ -61,6 +61,7 @@ openstack_host_packages:
   - irqbalance
   - libkmod-dev
   - libkmod2
+  - linux-image-extra-virtual
   - lvm2
   - python-software-properties
   - python-dev