Fix kernel post-installation script

`/etc/kernel/postinst.d/nova-kernel-permissions.sh` (introduced to fix Bug #1507915) is supposed to make newly installed kernels readable to the nova user, as kernels on an Ubuntu system are otherwise only readable to the root user [0].

This script didn't work for a few reasons:

- It never ran, because scripts in `/etc/kernel/postinst.d` are called by `run-parts`, and run-parts skips any script with a period in the name [1].
- Its shebang was missing its bang
- If installation of the same kernel is installed more than once (e.g. reinstallation), `dpkg-statoverride` (and the whole kernel installation) would exit with error, complaining about an override already existing [2].

Fixed with these changes respectively:
- Renamed script to remove the period
- Fixed typo in shebang
- Added `--force` flag to `dpkg-statoverride`

[0] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725
[1] https://bugs.launchpad.net/ubuntu/+source/debianutils/+bug/38022
[2] https://bugs.launchpad.net/openstack-manuals/+bug/1275080

Change-Id: I0e130e3c3ecf2171dbdc0e9a809f8066c30d4bc9
Closes-Bug: 1763479

files/nova_kernel_permissions

@@ -1,4 +1,4 @@
-#/bin/sh
+#!/bin/sh
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -17,4 +17,4 @@ version="$1"
 if [ -z "$version" ]; then
   exit 0
 fi
-exec dpkg-statoverride --update --add root nova 0640 "/boot/vmlinuz-${version}"
+exec dpkg-statoverride --force --update --add root nova 0640 "/boot/vmlinuz-${version}"

tasks/drivers/kvm/nova_kernel_permissions.yml

@@ -34,6 +34,6 @@
 
 - name: Script installed to make future kernels readable to nova group/user
   copy:
-    src: 'nova_kernel_permissions.sh'
-    dest: '/etc/kernel/postinst.d/nova_kernel_permissions.sh'
+    src: 'nova_kernel_permissions'
+    dest: '/etc/kernel/postinst.d/nova_kernel_permissions'
     mode: '0755'