Wire up LoadBalancerService

* Add to the schema validation.
* Add integration tests, as per the NatService test (to be DRYed out later)
* Wire up in the EdgeGatewayConfiguration class.

Diffing is not yet handled. There is a known issue with this due to the VSE
adding a 'dynamic' :Operational field.
Enable LoadBalancer Differ tests

Failing for now, due to:

* 'Operational: false' Pool element
* 'Uri' appears to come back from the API for ServicePorts, despite being absent
  in previous retrievals :(
'Fix' for the HealthCheck Uri

The integration tests are highlighting that the API adds a Uri value
to all ServicePort HealthCheck entries. This conflicts with discoveries
from taking output from LoadBalancer setup via the UI.

Joy.

For now, treating API-based updates as the winners, since this is how this
tool is designed to behave.
Add LoadBalancerConfigurationDiffer

LoadBalancerService adds a per-Pool 'Operational' element, which
technically is dynamic and so we cannot account for it in the
generator.

LoadBalancerConfigurationDiffer handles stripping this element out
from the remote config before comparison.
Fix failing unit tests due to Uri (re)addition
Add unit tests for LoadBalancerConfigurationDiffer

This also highlighted that we were displaying the wrong output from the
diff: we were testing diff ok, but showing the HashDiff of the unstripped
destination
Merge pull request #159 from alphagov/64583056_wire_up_load_balancer

[#64583056] Wire up LoadBalancerService management