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/manager.py b/tempest/scenario/manager.py
index 2d8935e..0fe15a3 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -706,17 +706,14 @@
                         network['id'])
         return network
 
-    def _create_subnet(self, network, subnets_client=None,
-                       routers_client=None, namestart='subnet-smoke',
-                       **kwargs):
+    def create_subnet(self, network, subnets_client=None,
+                      namestart='subnet-smoke', **kwargs):
         """Create a subnet for the given network
 
         within the cidr block configured for tenant networks.
         """
         if not subnets_client:
             subnets_client = self.subnets_client
-        if not routers_client:
-            routers_client = self.routers_client
 
         def cidr_in_use(cidr, tenant_id):
             """Check cidr existence
@@ -1146,12 +1143,11 @@
             router = self._get_router(client=routers_client,
                                       tenant_id=tenant_id)
             subnet_kwargs = dict(network=network,
-                                 subnets_client=subnets_client,
-                                 routers_client=routers_client)
+                                 subnets_client=subnets_client)
             # use explicit check because empty list is a valid option
             if dns_nameservers is not None:
                 subnet_kwargs['dns_nameservers'] = dns_nameservers
-            subnet = self._create_subnet(**subnet_kwargs)
+            subnet = self.create_subnet(**subnet_kwargs)
             if not routers_client:
                 routers_client = self.routers_client
             router_id = router['id']
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)
 
diff --git a/tempest/scenario/test_network_v6.py b/tempest/scenario/test_network_v6.py
index b687aa0..210f0ba 100644
--- a/tempest/scenario/test_network_v6.py
+++ b/tempest/scenario/test_network_v6.py
@@ -78,9 +78,9 @@
         if dualnet:
             network_v6 = self._create_network()
 
-        sub4 = self._create_subnet(network=network,
-                                   namestart='sub4',
-                                   ip_version=4)
+        sub4 = self.create_subnet(network=network,
+                                  namestart='sub4',
+                                  ip_version=4)
 
         router = self._get_router()
         self.routers_client.add_router_interface(router['id'],
@@ -93,11 +93,11 @@
         self.subnets_v6 = []
         for _ in range(n_subnets6):
             net6 = network_v6 if dualnet else network
-            sub6 = self._create_subnet(network=net6,
-                                       namestart='sub6',
-                                       ip_version=6,
-                                       ipv6_ra_mode=address6_mode,
-                                       ipv6_address_mode=address6_mode)
+            sub6 = self.create_subnet(network=net6,
+                                      namestart='sub6',
+                                      ip_version=6,
+                                      ipv6_ra_mode=address6_mode,
+                                      ipv6_address_mode=address6_mode)
 
             self.routers_client.add_router_interface(router['id'],
                                                      subnet_id=sub6['id'])