Fix neutron subnets with empty values.

Subnet was expended with ability to set and update parameters like
specifing dns servers, allocation pools and also specific host routes.
Sadly it introduced issue when calling neutron_subnet without specifying
values for these parameters.
Closes-Bug: #1312628

Change-Id: Id1207ab793fc65c43c10afcfe2271e2c02a912d3
(cherry picked from commit 78428c844f73bf585a8f8a3bdf615ba2a0e8983b)