feature: allow to add additional regions outside of the same Juju model/controller

Horizon can handle multiple regions by adding AVAILABLE_REGIONS in local_settings.py. And the charm has ability to add regions through relation data.

However, if two keystones are on a separate Juju controller (it's typical in geographically separated multi-region story), we cannot add regions each other. It would be nice if the charm accepts additional region info from outside of the same Juju model/controller.

[templates/ocata/local_settings.py]
# For multiple regions uncomment this configuration, and add (endpoint, title).
#AVAILABLE_REGIONS = [
# ('http://cluster1.example.com:5000/v2.0', 'cluster1'),
# ('http://cluster2.example.com:5000/v2.0', 'cluster2'),
#]
{% if regions|length > 1 -%}
AVAILABLE_REGIONS = [
{% for region in regions -%}
    ('{{ region.endpoint }}', '{{ region.title }}'),
{% endfor -%}
]
{% endif -%}

OPENSTACK_HOST = "{{ service_host }}"
OPENSTACK_KEYSTONE_DEFAULT_ROLE = "{{ default_role }}"
{% if api_version == "3" -%}
OPENSTACK_KEYSTONE_URL = "{{ service_protocol }}://%s:{{ service_port }}/v3" % OPENSTACK_HOST
OPENSTACK_API_VERSIONS = { "identity": 3, }
OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT = True
OPENSTACK_KEYSTONE_DEFAULT_DOMAIN = "{{ admin_domain_id }}"
{% else -%}
OPENSTACK_KEYSTONE_URL = "{{ service_protocol }}://%s:{{ service_port }}/v2.0" % OPENSTACK_HOST
{% endif -%}