hook failed: "certificates-relation-changed" for vault:certificates due to SSLCertificateFile: file '/etc/apache2/ssl/keystone/cert_XXX' does not exist or is empty

Steps to reproduce:
1. Deploy https://jujucharms.com/openstack-base/ modified to use next charms
2. Add Vault and initialize according to https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/rocky/app-vault.html
3. Add certificate lifecycle management according to https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/rocky/app-certificate-management.html

The Keystone charm will never finish the process.

In the logs you will find this:
2019-04-03 07:08:28 DEBUG certificates-relation-changed Enabling site openstack_https_frontend.
2019-04-03 07:08:28 DEBUG certificates-relation-changed To activate the new configuration, you need to run:
2019-04-03 07:08:28 DEBUG certificates-relation-changed systemctl reload apache2
2019-04-03 07:08:28 DEBUG certificates-relation-changed Job for apache2.service failed because the control process exited with error code.
2019-04-03 07:08:28 DEBUG certificates-relation-changed See "systemctl status apache2.service" and "journalctl -xe" for details.

journalctl shows this:
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 systemd[1]: apache2.service: Failed to reset devices.list: Operation not permitted
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 systemd[1]: Starting The Apache HTTP Server...
-- Subject: Unit apache2.service has begun start-up
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- Unit apache2.service has begun starting up.
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 apachectl[56435]: AH00526: Syntax error on line 8 of /etc/apache2/sites-enabled/openstack_https_frontend.conf:
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 apachectl[56435]: SSLCertificateFile: file '/etc/apache2/ssl/keystone/cert_172.16.122.10' does not exist or is empty
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 apachectl[56435]: Action 'start' failed.
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 apachectl[56435]: The Apache error log may have more information.
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 systemd[1]: apache2.service: Control process exited, code=exited status=1
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 systemd[1]: apache2.service: Failed with result 'exit-code'.
Apr 03 07:08:28 juju-7aa29b-3-lxd-1 systemd[1]: Failed to start The Apache HTTP Server.
-- Subject: Unit apache2.service has failed
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- Unit apache2.service has failed.
--
-- The result is RESULT.

Manually starting apache2 will resolve the issue.

This appears to be a race condition where we attempt to (re)start the Apache process before the certificate data is written to disk.