Remove routers_client from manager._create_subnet

This is to:
1. remove routers_client from manager._create_subnet
   because routers_client is not used for creating subnet.
2. rename _create_subnet to create_subnet for it is used
   by several testcases.

Change-Id: I102c3b01abe0ae752196d27893d43e64acfb57a6
diff --git a/tempest/scenario/test_network_basic_ops.py b/tempest/scenario/test_network_basic_ops.py
index 1c4e262..338345a 100644
--- a/tempest/scenario/test_network_basic_ops.py
+++ b/tempest/scenario/test_network_basic_ops.py
@@ -231,10 +231,10 @@
     def _create_new_network(self, create_gateway=False):
         self.new_net = self._create_network()
         if create_gateway:
-            self.new_subnet = self._create_subnet(
+            self.new_subnet = self.create_subnet(
                 network=self.new_net)
         else:
-            self.new_subnet = self._create_subnet(
+            self.new_subnet = self.create_subnet(
                 network=self.new_net,
                 gateway_ip=None)