Allow os-{type}-hostname to also override protocol

[Billy Olsen]

When deploying a proxy for OpenStack services the proxy itself may provide the SSL transport rather than the OpenStack services themselves. In such a deployment, SSL is not configured nor managed by the OpenStack service components (keystone, cinder, nova-compute, etc) and instead the proxy is handling the SSL traffic.

Rather than introduce yet another config option, a simple change to the way the os-{endpoint_type}-hostname options is treated will allow the user to also specify that the endpoints should be advertised as SSL and that an external proxy will manage the SSL configuration for them.

As such, this change would allow the user to specify the following options:

1. Override the hostname advertised, let the charms themselves manage the SSL

    juju set <service> os-public-hostname='foobar.example.org'

2. Override the hostname advertised and the URL scheme.

    juju set <service> os-public-hostname='https://foobar.example.org'

In the second configuration option, the public endpoints for the service will be advertised using https for the scheme/protocol and foobar.example.org for the address. The path portion of the endpoint URL will continue to be provided by the specific charm.

[Junien F (axino)]

I'd like to have this as well.