Make dns_servers option deprecated

CONF.network.dns_servers is not used anywhere, so this is to deprecate it.

Change-Id: I60ecad0e58394dcf912164095922276f4340f90b
diff --git a/releasenotes/notes/deprecate-dns_servers-option-0xf2f297ee47a5ff.yaml b/releasenotes/notes/deprecate-dns_servers-option-0xf2f297ee47a5ff.yaml
new file mode 100644
index 0000000..30551cb
--- /dev/null
+++ b/releasenotes/notes/deprecate-dns_servers-option-0xf2f297ee47a5ff.yaml
@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    The config option ``CONF.network.dns_servers`` is no longer used
+    anywhere, so it is deprecated and will be removed in the future.
+
diff --git a/tempest/config.py b/tempest/config.py
index fbe18a3..897a5ce 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -685,7 +685,10 @@
     cfg.ListOpt('dns_servers',
                 default=["8.8.8.8", "8.8.4.4"],
                 help="List of dns servers which should be used"
-                     " for subnet creation"),
+                     " for subnet creation",
+                deprecated_for_removal=True,
+                deprecated_reason="This config option is no longer "
+                                  "used anywhere, so it can be removed."),
     cfg.StrOpt('port_vnic_type',
                choices=[None, 'normal', 'direct', 'macvtap'],
                help="vnic_type to use when launching instances"