Improve the help text for cells options (7)

Some comments on `Improve the help text for cells options`
reviews are implemented with this patch.

- nova.cells.opts:list_ops function removed.

Implements blueprint centralize-config-options-newton
Change-Id: Idcaf66f0db1695d4fff34a656df2934c1b6d2ede

nova/cells/opts.py

@@ -26,7 +26,3 @@ def get_cell_type():
     if not CONF.cells.enable:
         return
     return CONF.cells.cell_type
-
-
-def list_opts():
-    return []

setup.cfg

@@ -28,7 +28,6 @@ packages =
 oslo.config.opts =
     nova = nova.opts:list_opts
     nova.conf = nova.conf.opts:list_opts
-    nova.cells = nova.cells.opts:list_opts
     nova.compute = nova.compute.opts:list_opts
     nova.network.neutronv2 = nova.network.neutronv2.api:list_opts
     nova.virt = nova.virt.opts:list_opts