Move add/remove fixed ip action to servers_client

Add/remove fixed ips are server action and currently in
interface_client. As all other servers actions are in servers_client
so these two methods also should be in servers_client instead of
interface_client.

This commit moves add/remove fixed ips methods to servers_client.

Partially implements blueprint consistent-service-method-names

Change-Id: Idacc612bfed3d5a4abd58c8a987f68675b948f86
diff --git a/tempest/api/compute/servers/test_attach_interfaces.py b/tempest/api/compute/servers/test_attach_interfaces.py
index 530e422..24d503f 100644
--- a/tempest/api/compute/servers/test_attach_interfaces.py
+++ b/tempest/api/compute/servers/test_attach_interfaces.py
@@ -186,7 +186,7 @@
         self.assertTrue(interface_count > 0)
         self._check_interface(ifs[0])
         network_id = ifs[0]['net_id']
-        self.client.add_fixed_ip(server['id'], networkId=network_id)
+        self.servers_client.add_fixed_ip(server['id'], networkId=network_id)
         # Remove the fixed IP from server.
         server_detail = self.os.servers_client.show_server(
             server['id'])['server']
@@ -199,4 +199,4 @@
                     break
             if fixed_ip is not None:
                 break
-        self.client.remove_fixed_ip(server['id'], address=fixed_ip)
+        self.servers_client.remove_fixed_ip(server['id'], address=fixed_ip)