Do not install linux-image-extra by default

In Ubuntu 14.04, the kernel shipped was lightweight, and some
extra features like using vhost_net, required the package
linux-image-extra.

In Ubuntu 16.04 and above, this is not the case anymore,
as many kernel packages ship with extra features.

vhost_net is generally shipped with the kernel package nowadays
and was the only one documented for the addition of the
linux-image-extra into our roles.

Change-Id: I579484ccefe0f29c40f8fed4b2d1b73aaeb917b1
Closes-Bug: 1783342
(cherry picked from commit 8c46d18988)

vars/ubuntu-16.04.yml

@@ -64,7 +64,6 @@ openstack_host_distro_packages:
   - irqbalance
   - libkmod-dev
   - libkmod2
-  - linux-image-extra-{{ ansible_kernel }}
   - lvm2
   - python-software-properties
   - python-dev