[stable-only] Add writer DB context to "add_provisioning_component"

To ensure the "standardatributes" register is present during the
creation of the "provisioningblocks" register, the method
"add_provisioning_component" is wrapped with a database writer
context. That will ensure the standard attribute ID, used as a
foreign key in the "provisioningblocks" table, is present during
the transaction.

Closes-Bug: #1991222
Change-Id: If57d822ff617c2f9b5e5cade7d4ca74065376e55

Author: ralonsoh

neutron/db/provisioning_blocks.py

@@ -42,6 +42,7 @@ def add_model_for_resource(resource, model):
 
 
 @db_api.retry_if_session_inactive()
+@db_api.CONTEXT_WRITER
 def add_provisioning_component(context, object_id, object_type, entity):
     """Adds a provisioning block by an entity to a given object.