Improve the help text for cells options (8)

- Deprecated note added for cells.driver option.
- Reno added for the deprecated option cells.driver.

Implements blueprint centralize-config-options-newton

Change-Id: I2addaa18cc9cbdbbcc90f2e514023ebf826d82f0

nova/conf/cells.py

@@ -363,8 +363,10 @@ Related options:
 cell_manager_opts = [
         cfg.StrOpt('driver',
                 default='nova.cells.rpc_driver.CellsRPCDriver',
-                help="""
-Cells communication driver
+                deprecated_for_removal=True,
+                deprecated_reason='The only available driver '
+                                  'is the RPC driver.',
+                help="""Cells communication driver
 
 Driver for cell<->cell communication via RPC. This is used to
 setup the RPC consumers as well as to send a message to another cell.

releasenotes/notes/deprecate-cells-driver-options-473893e4e87f95c2.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    The cells.driver configuration option is now deprecated and
+    will be removed at Ocata cycle.