Switch BaseAdminNetworkTest on test_routers_dvr

test_routers_dvr doesn't use BaseRouterTest specific methods at all,
so it is not necessary to inherit from BaseRouterTest. This patch
removes such unnecessary dependency for simple inheritance.

Change-Id: Ic9d741f64128e4532c2206a57954c7d1958bacef
diff --git a/tempest/api/network/admin/test_routers_dvr.py b/tempest/api/network/admin/test_routers_dvr.py
index 4ccad30..f9a0cfb 100644
--- a/tempest/api/network/admin/test_routers_dvr.py
+++ b/tempest/api/network/admin/test_routers_dvr.py
@@ -15,13 +15,13 @@
 
 import testtools
 
-from tempest.api.network import base_routers as base
+from tempest.api.network import base
 from tempest.lib.common.utils import data_utils
 from tempest.lib import decorators
 from tempest import test
 
 
-class RoutersTestDVR(base.BaseRouterTest):
+class RoutersTestDVR(base.BaseAdminNetworkTest):
 
     @classmethod
     def resource_setup(cls):