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: I70fa446c2dd00e12702280388299d189e0f3e5c4
Depends-On: https://review.openstack.org/#/c/585376/
Closes-Bug: 1783342

deploy-guide/source/targethosts-prepare.rst

@@ -56,6 +56,12 @@ Configure Ubuntu
        # apt-get install bridge-utils debootstrap ifenslave ifenslave-2.6 \
          lsof lvm2 ntp ntpdate openssh-server sudo tcpdump vlan python
 
+#. Install the kernel extra package if you have one for your kernel version \
+
+   .. code-block:: shell-session
+
+       # apt install linux-image-extra-$(uname -r)
+
 #. Add the appropriate kernel modules to the ``/etc/modules`` file to
    enable VLAN and bond interfaces:
 

tests/roles/bootstrap-host/vars/ubuntu.yml

@@ -25,7 +25,6 @@ packages_install:
   - iptables
   - iputils-tracepath
   - ipython
-  - linux-image-extra-{{ ansible_kernel }}
   - lvm2
   - parted
   - python2.7