Change fixed/floating ip clients to return one value and update tests

Add ResponseBodyData for cases like delete_floating_ips_bulk where a
string is returned as the body. The client has to dereference the data
but I think this is a bit cleaner than having some methods return two values.

Partially implements: blueprint clients-return-one-value

Change-Id: I87b092b2dd2a231049fc39312038f2a2d5101028
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index b1f5cce..07c06df 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -512,7 +512,7 @@
         Nova clients
         """
 
-        _, floating_ip = self.floating_ips_client.create_floating_ip(pool_name)
+        floating_ip = self.floating_ips_client.create_floating_ip(pool_name)
         self.addCleanup(self.delete_wrapper,
                         self.floating_ips_client.delete_floating_ip,
                         floating_ip['id'])