Ensure that linux-image-extra matches running kernel

When the host is installed, the loading of the kernel module
'vhost_net' may fail due to the module not being installed.

Currently the role installs the 'linux-image-extra-virtual'
package which will install the latest kernel and latest
extras package, but this may not match the running kernel on
the host.

This results in the following error when trying to load the
vhost_net module:

modprobe: FATAL: Module vhost_net not found.

This patch ensures that the linux-image-extra package
installed matches that of the running kernel.

Change-Id: I08271fcdfae3977f202ad7d6b03f61ab00e64370

vars/ubuntu-14.04.yml

@@ -69,7 +69,7 @@ openstack_host_distro_packages:
   - irqbalance
   - libkmod-dev
   - libkmod2
-  - linux-image-extra-virtual
+  - linux-image-extra-{{ ansible_kernel }}
   - lvm2
   - python-software-properties
   - python-dev

vars/ubuntu-16.04.yml

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