API "links" may end up with the wrong host/port

When enabled_host_header is enabled there is the possibility that the "links" in API responses will get the wrong URL path back.

For example, in a devstack run, a link was rendered:
        "links": {
            "self": "http://127.0.0.1:60053/v2/zones/a3365b47-ee93-43ad-9a60-2b2ca96b1898/shares/4495ffbb-b7d1-43e0-9423-f0a4172e5f9e",

Where the port used is actually the uwsgi port and not the front end location as published in endpoints: http://10.21.21.126/dns

It seems like the api_base_uri is being ignored here and the "requests.host_url" may not be the right information to collect when running behind uwsgi.

This is the method used to generate the "link" host information. (_get_base_url)
https://github.com/openstack/designate/blob/master/designate/objects/adapters/api_v2/base.py#L77