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
Merge "Actually raise BadRequest if create_subnet fails"