Empty keystone.conf apache2 vhost

Normally the keystone.conf is removed to make sure the
default configuration provided in the keystone package
for Ubuntu is skipped. This will cause issues when upgrading
the keystone package since the keystone.postinst script when
upgrading will look for that file. [1]

Instead we should make sure this file is empty which will make
sure upgrading the package is possible without manual intervention.

This does not affect gate per se but will act as a guideline on how
it should be used.

[1] https://bugs.launchpad.net/ubuntu/+source/keystone/+bug/1737697

Closes-Bug: #1737697
Change-Id: Ia69a11614a1fdd32a4903b47427fe3e6a48f9205

manifests/keystone.pp

@@ -103,14 +103,17 @@ class openstack_integration::keystone (
     ssl_cert        => $::openstack_integration::params::cert_path,
     workers         => 2,
   }
-  # Workaround to purge Keystone vhost that is provided & activated by default with running
-  # Canonical packaging (called 'keystone').
+  # Workaround to empty Keystone vhost that is provided & activated by default with running
+  # Canonical packaging (called 'keystone'). This will make sure upgrading the package is
+  # possible, see https://bugs.launchpad.net/ubuntu/+source/keystone/+bug/1737697
   if ($::operatingsystem == 'Ubuntu') and (versioncmp($::operatingsystemmajrelease, '16') >= 0) {
     ensure_resource('file', '/etc/apache2/sites-available/keystone.conf', {
-      'ensure'  => 'absent',
+      'ensure'  => 'present',
+      'content' => '',
     })
     ensure_resource('file', '/etc/apache2/sites-enabled/keystone.conf', {
-      'ensure'  => 'absent',
+      'ensure'  => 'present',
+      'content' => '',
     })
 
     Package['keystone'] -> File['/etc/apache2/sites-available/keystone.conf']