* add support for relating with subordinate charms providing Service
Provider functionality via apache2 authentication modules;
* enable additional authentication methods on the keystone side to
accept parsed assertion data provided via apache2 authentication module
variables exported to WSGI environment;
* move https frontend and WSGI API apache config files to keystone
instead of relying on charm-helpers as modifications are needed there to
add IncludeOptional directives. openstack_https_frontend.conf is added
on purpose as ServerName cannot be correctly determined after ProxyPass
which results in TLS errors during SAML exchange process;
* add an additional relation to openstack-dashboard to provide URL
information necessary to trust 'origin' parameter in WebSSO URLs used by
horizon during the authentication process. Also add a context to render
the federation section that is used to render this information in
keystone.conf;

Subordinates can choose to use different apache2 authentication modules.
If those modules support vhost-level variables then multiple
subordinates for the same module can be used. For example,
mod_auth_mellon can be used multiple times in different vhosts to
protect federated token endpoints related to different identity provider
and protocol combinations).

Trusted dashboard relation could be used to provide dashboard origin URL
from a different site via cross-model relations.

NOTE: this functionality will be triggered only on Ocata+ (inclusive)

Change-Id: I1ef623b0b0e2a9f68cec4be550965c5e15e5f561