Include load_balancer in configuration unit tests

LoadBalancerService added to initial 'all configurations are changed' test.

More cases to be come...
Add load balancer to 'nothing changed' config test

'all configs are present but haven't changed' now includes LoadBalancer
DRY out edge_gateway_configuration test
fix indentation
Specific load_balancer-only update test
Update existing tests to include load_balancer in context
Add load_balancer-only test
Add "two things have changed" test

... to ensure logic is ok in not-1, not-all case
Fix mock data in EdgeGatewayConfiguration test

The test double of Core::EdgeGateway.vcloud_gateway_interface_by_id
is hardcoded to return a particular Interface section.

Make the inputs for tests match this double, otherwise the tests don't
really make sense.

This limitation with the mocking is addressed when we move to dependency
injection for the generators.
Add load balancer to test context

Missed one of the tests when adding the LoadBalancerService
Further clarification of different test data

... for LoadBalancerService
Add tests for 'no remote config' case; fix bug

These tests assume the case where there is a missing remote
configuration for a given service, for which we are subsequently
trying to configure.

Highlighted a NilClass exception in LoadBalancerConfigurationDiffer,
which is fixed in this commit.
Add test that missing remote service is ok

Specifically, test that we can update the NatService when we
there is no remote LoadBalancerService specified.
Merge pull request #7 from alphagov/load_balancer_general_case_tests

[#64583056] Load balancer general case tests