Rally isn't work with nested Heat templates

If we try to use heat with nested templates then Rally should parse external template at and get from it nested templates. The it should parse them as files to https://github.com/openstack/rally/blob/master/rally/plugins/openstack/scenarios/heat/utils.py#L120
 To reproduce that you need to run

 HeatStacks.create_and_delete_stack:
   -
     args:
       template_path: "nested_template.yaml"
     runner:
       type: "serial"
       times: 5
       concurrency: 1
     context:
       users:
         tenants: 1
         users_per_tenant: 1

Looks like we need to add one more convert too scenario for now to be able to use it like:

 HeatStacks.create_and_delete_stack:
   -
     args:
       template_path: "nested_template.yaml"
       files: resource.yaml
     runner:
       type: "serial"
       times: 5
       concurrency: 1
     context:
       users:
         tenants: 1
         users_per_tenant: 1

But the next step would be to add to Rally ability to pass nested templates into the `files` variable in-box. Possibly this would be more user friendly.