Mount /boot from the host within the nova-compute container

libguestfs expects to find /boot/vmlinuz-* for the running version of
the kernel. This check is duplicated in nova-compute when libguestfs has
failed to launch, providing a useful bread crumb for operators [1].

Obviously when this is called from within the nova-compute container in
the context of a TripleO deployment this can easily fail after a minor
update that has pulled in a newer container containing a newer kernel.
This check could also fail in the opposite case if the host kernel is
updated past the version present in the container.

This change works around this by simply passing /boot as read-only
through to the nova-compute container ensure libguestfs is able to
always find the correct version of vmlinuz.

This should also allow us to eventually drop the kernel RPM from the
nova-compute container that has been a constant source of maintenance
overhead in terms of CVEs etc.

[1] aa096fd183/nova/virt/disk/vfs/guestfs.py (L75-L97)

Conflicts:
    deployment/nova/nova-compute-container-puppet.yaml

Change-Id: Iadef8f3300bb1b5b995052c1a35a1becbfd5730c
(cherry picked from commit 9c5c36632d)
(cherry picked from commit 02faf53ea8)
(cherry picked from commit 1f5c51d5d0)
(cherry picked from commit bb27721165)

docker/services/nova-compute.yaml

@@ -250,6 +250,7 @@ outputs:
                       - /var/lib/libvirt:/var/lib/libvirt
                       - /sys/class/net:/sys/class/net
                       - /sys/bus/pci:/sys/bus/pci
+                      - /boot:/boot:ro
                 environment:
                   list_concat:
                     - {get_param: NovaComputeOptEnvVars}