Actually raise BadRequest if create_subnet fails

Second attempt at fixing a problem in the Tempest
network test base setup where it tries to create
a subnet and if it fails it needs to raise a
BadRequest rather than an UnboundLocalError from
trying to use the body variable before it's declared.

Fixes bug 1192767

Change-Id: I2c97723b1056ae34e68a5acb5283c291a5dcfad1
diff --git a/tempest/api/network/base.py b/tempest/api/network/base.py
index 8068284..03e73df 100644
--- a/tempest/api/network/base.py
+++ b/tempest/api/network/base.py
@@ -73,6 +73,7 @@
         cidr = netaddr.IPNetwork(cls.network_cfg.tenant_network_cidr)
         mask_bits = cls.network_cfg.tenant_network_mask_bits
         # Find a cidr that is not in use yet and create a subnet with it
+        body = None
         failure = None
         for subnet_cidr in cidr.subnet(mask_bits):
             try: