Allow empty hashes for LoadBalancer services

All parameters underneath the per-service entries for
virtual_server service_profile and pool service entries have
defaults, thereby allowing an empty hash as an argument.

The schema prevented this though, which this commit fixes.

Now you can specify:

pools:
  name: test-pool
  service:
    http: {}
virtual_servers:
  name: test-vs
  service_profile:
    http: {}
Merge pull request #22 from alphagov/66950180_empty_services

[#66950180] Allow empty hashes for LoadBalancer services