Add tests for empty pool/virtual_server combos

* VirtualServers cannot be specified without a matching Pool. Test currently
  relies on vCloud telling us this (which is relatively quick and informative)
* Pools can be specified alone, and this is a valid case when setting up a
  LoadBalancer.
* We should be able to clear out a LoadBalancer by specifying no Pools or
  VirtualServers
* In the process, clean up the expectations of pool/virtual_server
  in the schema to match our requirements.
Merge branch 'master' into additional_load_balancer_tests
need microseconds to prevent temp file collision
Clarify test purpose and fix line lengths
Add test for missing VirtualServer pool attribute
Merge pull request #4 from alphagov/additional_load_balancer_tests

[#64583056] Additional load balancer tests for Pool/VirtualServer combinations