Avoid default quota limit of 10 subnets

This test class only fails with no tenant isolation in the gate. The only
reason I can think of for this is that the non-isolated demo tenant has
an extra subnet created by devstack whereas the isolated tenant for this
test class does not. I tried putting addCleanups for the network and subnets
in the for loop but that caused other strange problems.

Change-Id: Ib86d54ee4cf317bb9d93d28fd62f44d738b5efaa
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index 78b51c8..fa7fa64 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -275,7 +275,7 @@
         next_cidr = netaddr.IPNetwork(self.tenant_cidr)
         # Prepare to build several routes
         test_routes = []
-        routes_num = 5
+        routes_num = 4
         # Create a router
         self.router = self._create_router(
             data_utils.rand_name('router-'), True)