Split the endpoint_type for signalling and heatclient

[Ethan Lynn]

For now, the resources (ASG/SoftwareDdeployment/HeatStack) which related to heatclient will refer to endpoint_type in [clients_heat] to generate URL, most of time we set it to publicURL. This will result that the URL push to VM use publicURL endpoint, which is acceptable, and also the OS::Heat::Stack will use publicURL too.

In some cuostomers' env, they might not have public connection between heat services in multi region env, so they want to use internalURL for OS::Heat::Stack but leave those resources which will accept signals from VM to use publicURL.

How about adding a [signals] sections which will be consume by SignalResponder class to generate heat url for VM to send signals?
Or any other way we can define endpoint_type for each resource type.

Ideas are welcome.