neutron: Don't use primitives in entities

Primitives are auto initialized to false, instead use boxed types to
represent values that need to be serialized.

Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
neutron: Test empty serialization of entities

To avoid further confusion with fields having default values, added test
for the serialization of an empty Port/Subnet/Network.

Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>