Mark test_dhcp_port_status_active api test as unstable

Since some time it is failing pretty often. See related bug for details.
So for now lets mark it as unstable to make gate litte bit more stable.

Change-Id: I4e7f8f4b445fa321c58c70d5ba7d8a8c96ee94a7
Related-bug: 1906654
diff --git a/neutron_tempest_plugin/api/admin/test_dhcp_agent_scheduler.py b/neutron_tempest_plugin/api/admin/test_dhcp_agent_scheduler.py
index d0adcb8..9dc4438 100644
--- a/neutron_tempest_plugin/api/admin/test_dhcp_agent_scheduler.py
+++ b/neutron_tempest_plugin/api/admin/test_dhcp_agent_scheduler.py
@@ -13,6 +13,7 @@
 #    under the License.
 
 from neutron_lib import constants
+from neutron_lib.utils import test
 from tempest.lib import decorators
 
 from neutron_tempest_plugin.api import base
@@ -33,6 +34,7 @@
         cls.cidr = cls.subnet['cidr']
         cls.port = cls.create_port(cls.network)
 
+    @test.unstable_test("bug 1906654")
     @decorators.idempotent_id('f164801e-1dd8-4b8b-b5d3-cc3ac77cfaa5')
     def test_dhcp_port_status_active(self):