[TEST] Enforce provided settings in test base classes

Enviorment variables might override the tests settings even if
they are explicitly set. Other base classes like InternalTestCluster
also specify `config.ignore_system_properties: true` to ensure `what
we set is what we get`
Test: make test less evil so it doesn't take so long
[TEST] Enforce that the node from ElasticsearchSingleNodeTest is alone in its cluster.

In particular, this removes the ability to create new node and adds the ability
to get a reference to the single node that the cluster is made of.
Docs: explain that SerialMergeScheduler just maps to CMS for back compat

Closes #6878
Mappings: Add transform to document before index.

Closes #6566