Support OpenStack Heat signal url

[ntk148v]

By now, Faythe only support alarm_url - A signed url to handle the alarm. The URL is signed with an AWS-style signature without any credentials come along. This is a potential security risk.

Faythe should support signal_url as well. This is a url to handle the alarm using native Heat API: https://docs.openstack.org/heat/latest/template_guide/openstack.html#OS::Heat::ScalingPolicy

Format: https://docs.openstack.org/api-ref/orchestration/v1/index.html?expanded=send-a-signal-to-a-resource-detail

/v1/{tenant_id}/stacks/{stack_name}/{stack_id}/resources/{resource_name}/signal