Version update to 1.6 of the service-path model

Version update to 1.6 of the service-path model

RPCs originally in service-path are now split in several modules which
expose them.
- Replacement of node-id by tp-node-id in path-description/resource to
avoid potential conflicts in the service handler code.
- Move previous and new service path yang models to the 'service_path'
folder.
- Rename transportpce yang models with their revision date.
- Add old revision statement to service_path_v1.6 yang files.
- Modifiy prefix import for transportpce-routing-constraints to be
'transportpce-routing-constraints' and not 'org-openroadm-routing
-constraints' in yang 'transportpce-common-service-path-types
@2017-10-16.yang'.
- Add revision-date statement to import.
- Add missing 'leaf status-message' in grouping rpc-response-status-ex
in 'transportpce-common-service-path-types@2017-10-16.yang'.
- Move stubpce and stubrenderer to tests folder.
- Add missing grouping 'stubpce-path-description' and container
'path-description-list' to transportpce-pce.yang model.

Change-Id: I06667dabb39490c378dfbbf4462c262e078a7f9d
Signed-off-by: Olivier <olivier.renais@orange.com>