Move 'zoning_mode' back to DEFAULT section

We moved 'zoning_mode' option to 'backend_defaults'
section since patch: [1]. But it has two issues:

1. It can not work because we still fetch the value
from the default section [2].
2. From the view of our current design. It's only
used in our framework code, which means it still can
not work if we overwrite the default value in one
specific backend section, such as:

[backend_defaults]
zoning_mode=fabric
[lvmbackend_1]
zoning_mode=other_value

Move this option back to DEFAULT section until we
need to.

Closes-Bug: #1724418

[1]: https://review.openstack.org/#/c/453654/
[2]: https://github.com/openstack/cinder/blob/master/cinder/zonemanager/utils.py#L56

Change-Id: I5b31082296f5bff5089d0f42a62d44bb009d177d

cinder/volume/manager.py

@@ -120,14 +120,15 @@ volume_manager_opts = [
                 default=False,
                 help='Offload pending volume delete during '
                      'volume service startup'),
+    cfg.StrOpt('zoning_mode',
+               help="FC Zoning mode configured, only 'fabric' is "
+                    "supported now."),
 ]
 
 volume_backend_opts = [
     cfg.StrOpt('volume_driver',
                default='cinder.volume.drivers.lvm.LVMVolumeDriver',
                help='Driver to use for volume creation'),
-    cfg.StrOpt('zoning_mode',
-               help='FC Zoning mode configured'),
     cfg.StrOpt('extra_capabilities',
                default='{}',
                help='User defined capabilities, a JSON formatted string '