Throw exception if 'name' is set for subnet_create

If suppose --name is provided by user, then below is the subnet_q dictionary
subnet_q = {u'name': u'crp', 'enable_dhcp': True, u'network_id': u'ed71df53-bf48-4b2d-8fa0-83a77c1d0f92', 'tenant_id': u'211d8662d6e24741949009525f08ced5', 'dns_nameservers': <object object at 0x7f870b1bc1a0>, 'gateway_ip': <object object at 0x7f870b1bc1a0>, 'contrail:ipam_fq_name': '', 'ipv6_ra_mode': <object object at 0x7f870b1bc1a0>, 'allocation_pools': <object object at 0x7f870b1bc1a0>, 'host_routes': <object object at 0x7f870b1bc1a0>, u'ip_version': 4, 'ipv6_address_mode': <object object at 0x7f870b1bc1a0>, u'cidr': u'13.0.0.0/24', u'network:tenant_id': u'211d8662d6e24741949009525f08ced5'}

If  --name option is not provided, then below is the subnet_q dictionary
subnet_q = {'name': '', 'enable_dhcp': True, u'network_id': u'ed71df53-bf48-4b2d-8fa0-83a77c1d0f92', 'tenant_id': u'211d8662d6e24741949009525f08ced5', 'dns_nameservers': <object object at 0x7f870b1bc1a0>, 'gateway_ip': <object object at 0x7f870b1bc1a0>, 'contrail:ipam_fq_name': '', 'ipv6_ra_mode': <object object at 0x7f870b1bc1a0>, 'allocation_pools': <object object at 0x7f870b1bc1a0>, 'host_routes': <object object at 0x7f870b1bc1a0>, u'ip_version': 4, 'ipv6_address_mode': <object object at 0x7f870b1bc1a0>, u'cidr': u'14.0.0.0/24', u'network:tenant_id': u'211d8662d6e24741949009525f08ced5'}


In both the cases, 'name' is present in the dictionary. I tested it.