Avoid possible conflicting KubernetesNode document

This avoids a tricky-to-debug situation where it can appear that
different labels (or even a different ip address) is applied to a host
than expected.

Change-Id: I29fc99581a85f9cd1275f5cc07dfcb1be0e98339

promenade/control/join_scripts.py

@@ -25,6 +25,8 @@ from promenade import policy
 
 LOG = logging.getLogger(__name__)
 
+SENTINEL = object()
+
 
 class JoinScriptsResource(BaseResource):
     """
@@ -48,6 +50,10 @@ class JoinScriptsResource(BaseResource):
         except exceptions.DeckhandException as e:
             raise falcon.HTTPInternalServerError(description=str(e))
 
+        if config.get_path('KubernetesNode:.', SENTINEL) != SENTINEL:
+            raise exceptions.ExistingKubernetesNodeDocumentError(
+                'Existing KubernetesNode documents found')
+
         node_document = {
             'schema': 'promenade/KubernetesNode/v1',
             'metadata': {

promenade/exceptions.py

@@ -243,6 +243,22 @@ class ApiError(PromenadeException):
     status = falcon.HTTP_400
 
 
+class ExistingKubernetesNodeDocumentError(ApiError):
+    """
+    An exception to represent the unexpected discovery of
+    `promenade/KubernetesNode/v1` documents in the given site design when
+    calling the `join-scripts` api.
+
+    **Message:** *Existing KubernetesNode documents found*
+
+    **Troubleshoot:** *KubernetesNode documents should not be in the site
+    design when using the join-scripts API for join script generation.*
+    """
+
+    title = 'Unexpected KubernetesNode document found'
+    status = falcon.HTTP_400
+
+
 class InvalidFormatError(PromenadeException):
     """
     An exception to cover invalid input formatting.