NSX|V: add configuration variable for dns_search_domain

Enable a admin to configure a global search domain. That is, if a
subnet is not created with a search domain (commit
d9f3ee826a) then is a domain is
defined in the configuration file then we can use that one.

In the nsxv section there will be a new variable
 - dns_search_domain

Change-Id: I112a00dbc89b1c7702e82ecfa6ec974b7b9cce8d

releasenotes/notes/dns-search-domain-configuration-a134af0ef028282c.yaml

@@ -0,0 +1,9 @@
+---
+prelude: >
+    Enable an admin to configure a global search domain.
+    This is used if no search domain is configured on a
+    subnet.
+features:
+  - A new configuration variable in the nsxv section will
+    enable the admin to configure a search domain. The new
+    variable is dns_search_domain.

vmware_nsx/common/config.py

@@ -580,6 +580,9 @@ nsxv_opts = [
                       "exclusive_router_appliance_size will be picked up if "
                       "--router-size parameter is not specified while doing "
                       "neutron router-create")),
+    cfg.StrOpt('dns_search_domain',
+               help=_("(Optional) Use this search domain if there is no "
+                      "search domain configured on the subnet.")),
     cfg.ListOpt('nameservers',
                 default=[],
                 help=_('List of nameservers to configure for the DHCP binding '

vmware_nsx/plugins/nsx_v/vshield/edge_utils.py

@@ -912,10 +912,14 @@ class EdgeManager(object):
             sub_binding = nsxv_db.get_nsxv_subnet_ext_attributes(
                 context.session,
                 subnet_id)
-            if sub_binding:
+            dns_search_domain = None
-                if sub_binding.dns_search_domain is not None:
+            if sub_binding and sub_binding.dns_search_domain:
-                    static_config['domainName'] = sub_binding.dns_search_domain
+                dns_search_domain = sub_binding.dns_search_domain
-                if sub_binding.dhcp_mtu:
+            elif cfg.CONF.nsxv.dns_search_domain:
+                dns_search_domain = cfg.CONF.nsxv.dns_search_domain
+            if dns_search_domain:
+                static_config['domainName'] = dns_search_domain
+            if sub_binding and sub_binding.dhcp_mtu:
                 static_config = self.add_mtu_on_static_binding(
                     static_config, sub_binding.dhcp_mtu)