Ensure HTTPS configuration completes

There was a race where the https apache2 site,
openstack_https_frontend.conf, would be rendered in one hook, then
subsequently the config-changed hook would run and enable that site.
However, the subsequent config-changed hook would see the template as
having not changed and therefore it would fail to restart apache2.
This lead to apache2 failing to listen on the correct ports.

This was due to CONFIGS.write_all() being called but a2ensite not
being called. This change fixes this race and adds a call to
configure_https() to ensure the configuration completes and apache2
is restarted.

Change-Id: I229d25c707a0630c9d609fd20a962a0de2e42c77
Closes-Bug: #1723892