Subnet create tests and minor network test updates

- Adding subnets POST tests
- Updating fixture assert methods to check partial
  names (starts with)
- Updating network tests setup to instantiate the expected
  network object
- Updating network test data admin_state_up param
- Updating network tests negative assertions
- Updating networks and subnets data to fixture
- Adding create_test_network and add_subnet_to_network
  fixture methods for tests set up methods
- Adding to fixture get data methods for dns_nameservers,
  allocation_pools and host_routes

Change-Id: I9462ffc907041af0f86046c80ab97c5872ef73ee

Conflicts:
	cloudroast/networking/networks/api/subnets/test_subnets_create.py