Add subnet tests for extra attributes

* Tests added for ipv4/6
test_create_delete_subnet_with_allocation_pools
test_create_delete_subnet_with_gw_and_allocation_pools
test_create_delete_subnet_with_host_routes_and_dns_nameservers
test_create_delete_subnet_with_dhcp_enabled
test_update_subnet_gw_dns_host_routes_dhcp
test_create_delete_subnet_all_attributes

* Adds helper to create/verify/delete subnet
* Adds helper to delete network

Implements: bp subnet-extra-attributes

Change-Id: I7c1b00f4fad797eb5dd904f1123b5bcd5e5af649
Merge "Add subnet tests for extra attributes"