EST-417: refactoring in line with latest ISIS-716 changes

- renamed CompositeFixtureScript -> DiscoverableFixtureScript
- SimpleFixtureScript is no more (execute etc moved up from CompositeFixtureScript into FixtureScript)
- this in readiness for refactoring towards fixtures that execute their own prerequisites.
Fixing error with existing fixture data (OXF leases...)

... should have HELLOWORLD as their landlord, not ACME (since OXF is owned by HELLOWORLD).
Marking the DocumentationContributions test to @Ignore

... since is just a spike, fails under certain circumstances (using a demo repository).
EST-417: refactoring and fixing failing tests

... main reason for failure is that findXxxByReference now throws exception; have introduced findXxxByReferenceElseNull where tests/Api expected null if not present.