Deprecate 'force_gateway_on_subnet' configuration option

Currently 'force_gateway_on_subnet' configuration is set to True
by default and enforces the subnet on to the gateway. With the
fix in https://review.openstack.org/#/c/233287/, gateway outside
the subnet can be added, and the configuration option now has
lost its significance.

With this patch, the configuration option is deprecated.
It should be removed in Newton release, and the system should
always allow gateway outside the subnet.
This patch is dependent on the fix for adding gateway outside
the subnet, mentioned above.

DocImpact: 'force_gateway_on_subnet' description should be
updated in the docs and marked as deprecated to be removed in
the Newton release.

Change-Id: I28b3d7add303ee479fc071c1de142b0f7811e4e5
Closes-Bug: #1335023
Closes-Bug: #1398768

neutron/common/config.py

@@ -144,9 +144,12 @@ core_opts = [
                       "services running on this machine must use the same "
                       "host value.")),
     cfg.BoolOpt('force_gateway_on_subnet', default=True,
+                deprecated_for_removal=True,
                 help=_("Ensure that configured gateway is on subnet. "
                        "For IPv6, validate only if gateway is not a link "
-                       "local address.")),
+                       "local address. Deprecated, to be removed during the "
+                       "Newton release, at which point the gateway will not "
+                       "be forced on to subnet.")),
     cfg.BoolOpt('notify_nova_on_port_status_changes', default=True,
                 help=_("Send notification to nova when port status changes")),
     cfg.BoolOpt('notify_nova_on_port_data_changes', default=True,

releasenotes/notes/deprecate-force_gateway_on_subnet-376855c4e66f4e11.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - The 'force_gateway_on_subnet' option is deprecated and
+    will be removed in the 'Newton' cycle.